I was taught by my pedantic mother that the word 'firstly' is erroneous. Secondly, thirdly and so on are fine - but not firstly. The word is 'first'. I've no idea whether this is valid or in any way justifiable, but it doesn't stop me flinching whenever Jezza uses 'firstly' on UC. Such things are bred in the bone...
As to the original query, I was taught 'what follows a colon delivers what was promised before it'. If you mentally add 'which is to say...' immediately after the colon, you're probably on firm ground. It's about saying the same thing differently, 'coming at it from a different angle'.
It had everything a bike really needs: a frame, two wheels, somewhere to sit and something to steer with.
It had everything a bike really needs: (which is to say) a frame, two wheels, somewhere to sit and something to steer with.
As to semi-colons, they're like commas' big brothers: used to link/divide two bits of a sentence, but specifically (and differently from a comma) bits which are in some way connected/related. Bigtwins's eg earlier in the thread being an excellent example:
Use them sparingly; less is more.
You could use a comma:
Use them sparingly, less is more.
...but because the two thoughts are so closely interelated, it would be, tho' not strictly 'wrong', certainly missing a trick.
But you could not use a colon:
Use them sparingly: less is more.
...because 'less is more' is not 'Use them sparingly' expressed differently; it is a different, tho' connected, thought (an aesthetic assertion, as against an injunction).
As to whether 'any of this matters, so long as we understand what's being said', well, at least in part, that begs the question. A lot of this stuff is precisely to help ensure that we do understand what's being said. All of it came about initially as an attempt to achieve exactly that: to enable us to express ideas and the like in ways that others would be able to understand. And while it's true that many ideas and the like can be expressed clearly with even the most basic punctuation (and, by the same token, vocabulary) the more complex the ideas, the more likely these more subtle 'tools' will prove helpful.
But does it matter whether people put an apostrophe in 'its'? Probably not. And so many do (and there's in truth so little justification for its absence) that it will probably be de rigeur in 50 years' time. So be it. Language changes - and that's all to the good. It shows it's alive!