The questions they ask can be quite trying. I'm speaking with experience from only US uni apps (since I haven't tried, nor am allowed to, apply for UK unis yet), but I believe that any good uni will also ask similar questions. They typically require you to search yourself, for your passions, your character, what makes you tick, your weaknessess, your strengths. It's all you, you, you. Very second-person-centric.
Since I've already been through 2 rounds of applications, I guess I have quite some experience to talk about it.
For the many who haven't actually thought and reflected enough about themselves over their years, this might actually be pretty shocking. You might begin with something in mind, then run out of ideas halfway through the essay, or perhaps realise that that wasn't what you really meant, or find out that you should be writing in a different style. Probably many corrections. Well, all I can say is: Start early. Yes, you might have heard this for any important task assigned, but in this case, it's particularly important. There'll be alot of thinking and reflecting if this is your first time.
Even for those who have done some self reflection, but never very in-depth should also beware, since although you will have an easier time than those who have never done this before, you might still end up re-writing some essays, or at least tweaking quite a bit. It will require time, but rush work is possible, compared to those who are doing for the first time. This is actually the category I fall into. I've done in-depth reflection before, but not in-depth enough, i.e. essay form with good vocabulary, grammar, and etc. Saying what you mean can be difficult at times, especially to get all the nuances right in the portrayal of yourself. Language can be a tricky thing.
I doubt anyone would actually fall into the category of having done enough in-depth reflection to complete all uni apps in one go with minimal corrections, so I shan't cover this part.
So well, yea, doing uni-apps actually helps you uncover more about yourself, especially since some questions will probably bring you to ponder about certain aspects you've never thought of before, or worse, always wanted to avoid. If you're really up for it, you could try taking a look and writing those essays for fun to discover more about yourself even if you're not applying for the uni. And if you do write those essays, and they turn out great, you might as well apply for the uni anyway since money is the only other thing that really stands in your way after that.