Knowing the difference between the two phonetic syllabaries of Japanese can be vital, when very much reading any mode or level of Japanese text.
On a surface level they are distinct, one is "squiggly" (ひらがな), and the other is more angular (カタカナ), both of which derive from using kanji as a method of writing out Japanese phonetically, Man'yougana, which then later developed through short-hand into their own twinned writing systems.
Hiragana is the primary one of the two, using nearly exclusively for writing out grammatical and syntactical components such as the particles: e.g. は (wa, topic particle), を ([w]o, direct object particle) and が (ga, subject particle). Each of these and more are as vital to the meaning and conveyance of a sentence as the words "and, is, with" and so on are in English, for example.
You can also write out regular words in hiragana for the sake of rendering words that might use either complex or obscure kanji much easier on the eyes.
Katakana's role is dual, on one hand it is used chiefly to write foreign words (i.e. コンピューター, konpyûtâ for "computer"), the names of non-Japanese people and places (i.e. デンマーク, Denmâku, "Danmark"). Secondly, it can be used for emphasis, by rendering a word into purely katakana it pops out, when a reader sees it, thus any ordinary word rendered in kanji or hiragana is accentuated in angular - akin to CAPITAL LETTERS in English.
Thus, my name Alex, would be written out in katakana as アレックス (arekkusu), approximating as closely as possible the sound of the name rather than a straight forward orthographic conversion.
I could say, my name is Alex, in informal Japanese:
僕の名前はアレックスだ
Boku no namae wa Arekkusu da
Using the kanji-spellings for the words boku ("I") and namae ("name").
In order to perhaps make the pronoun boku stand out all the more I could rewrite it as:
ボクの名前はアレックスだ
This is all pronounced in the same way as before, but the effect that it has on the reader of the sentence is that boku stands out, being written not just similarly to the name Alex, but some may perceive it as either childish or perhaps too informal, it nevertheless underlines a personality trait of sorts - making the pronoun stand out more.
Were I to render boku into ぼく, the result would be a much 'softer' and perhaps even more childish version of the pronoun due to its shape and writing it out as such, it could also be seen as much less intimidating and more amiable.
These things are ultimately dependent on the associations that people have with hiragana and katakana.
Associations with katakana tend to be "novel, foreign, imitative, emphasising, hard, simple, inorganic, fake, marked, young, male, futuristic, neutral, sharp, fresh, jarring, precise, angular" (Robertson), where it is precisely more youthful, novel and marked in its accentuation of making a thing such a pronoun stand out as in this case young and marked. Whereas hiragana has the qualities of "feminine, soft, smooth, round, tender, simple, childish, lovely, unmarked, intimate, private, nice, elegant, poetic, Japanese" (Robertson), where the visible graphical shape of the hiragana contrasts itself with katakana as being more tender and soft, if also childish.
Kanji in contrast to both of these has a more substantial form to it, in both meaning and shape thus it stands out all the more when compared to the two purely phonetic systems.
Case study - Ryu from Capcom's Street Fighter series:
Ryu from Capcom's fighting game franchise Street Fighter goes through all three of the writing systems when using his first person pronominal ore, during his appearance in the second game in the series, Street Fighter II, he purely uses the kanji form, 俺, as do the rest of the characters in the game who uses this masculine pronominal as their one of choice.
Once we enter the Zero (Alpha) series of games he switches to the hiragana form, おれ, presumably to underline his much softer and inexperienced personality compared to that of his friend and fellow martial artist, Ken, whose brashness and gutsy attitude has him use the kanji form, 俺, to put a contrast between the two characters.
Case in point:
Ken: よう、久しぶりだな！ 探すのに骨が折れたぜ。
You, hisashiburi da na! Sagasu no ni hone ga oreta ze.
Hey, long time since last! I’ve been breakin’ my arse looking for you.
Ryu:
……ケン、おれは……何を目指していたんだ……？
サガットを倒した、この拳……これが、真の格闘家の証なのか？
... Ken, ore wa... nani o mezashite ita n'da...?
Sagatto o taoshite, kono ken... kore ga, shin no kakutouka no akashi nano ka?
...Ken, what I have been searching for?
I defeated Sagat, this fist, surely that’s the proof of me being a true martial artist?
Ken：
おいおい……またいつもの悪いクセか？ 一発俺と勝負しろ、目を覚まさせてやるぜ！！
Oi oi.... mata itsumono warui kuze ka? Ippatsu ore to shoubu shiro, me o mezamasasete yaru ze!!
Come on now, always with the bad habits? Have round against me, then you’ll sober up, pal!
(Street Fighter Zero 2)
Please note that Japanese does not differentiate between I. me, mine and so on, these are merely artefacts in the English translation. As we can see Ryu appears also less confident than his boisterous friend, who also uses the emphatic particle ze, a very masculine and informal way of expressing assertion, akin to "pal" or in some cases it can equate to profanity.
As such it can have widely different conotations depending on who uses it and when.
Bibliography:
Street Fighter Zero 2 (ストリートファイターZERO2). Multiplatform. Capcom. 1996. Japan.
Robertson, Wes. Writing another's tongue - Orthographic Representations of Non-Fluency in Japanese Manga from pp. 161 - 178, in Manga Vision - Cultural and Communcative Perspectives. Monash University Publishing. 2016. Australia.