I was just checking out this forum while researching my next smartphone purchase and thought I'd answer even though you posted this a while ago...
Well, it's a 'horses for courses' kind of reason.
Firstly, it's good because you can install apps that need root access to do what they do. This doesn't mean you're ditching your HTC Sense UI, that requires rooting AND flashing another ROM and even then there are lots of developers out there creating custom ROMs with Sense included for those who want to keep it.
There are overclocking apps which also allow you to underclock and that can give you extra battery life. There are other automation apps; I read about someone who uses a pair of automation apps to get the best of both worlds with lots of frequent social network updates and e-mail/calendar syncs, but without leaving wifi or mobile data switched on and chewing his battery up constantly. Then there are backup apps like Titanium which I use a lot to roll back to previous good versions of apps when a new update doesn't work as well for whatever reason.
The next main reason is that it's the first step to installing a different ROM. Let me talk you through my Android history...
I bought an HTC Hero in Jan 2010 (paid in full, SIM free, unlocked). It had Android 1.5 on it with Sense. Sense was very highly rated even back then and it was one of the reasons I bought it over other makes with other (less well received) custom UIs.
Like many other Hero owners I waited and waited for an update to 2.1 and when I finally got it I was pretty pleased, but soon after I was tempted to root it for overclocking (and better battery life through underclocking) and Titanium backup.
Then I tried a 2.1 VillainROM with Sense ROM and was really chuffed. It felt a bit like having a new phone even compared to the official 2.1 ROM from HTC (which was a BIG step).
From there, I've just not looked back.
I next tried a pure Android 2.3 custom ROM from Feeyo/Cronos, effectively skipping 2.2 out and ditching Sense. I missed a couple of things about Sense, but it's really nice having a less bloated OS and being able to keep up with the core Android developments. And at the same time you can also get other built in features that developers include that make can make the experience even better than Google's latest.
I'm on my third custom ROM now (fifth ROM including the 1.5 release on the phone from day one and the official HTC 2.1 release).
Without this approach, my little old Hero would feel obsolete. As it is, it's still going strong (got 6.5 days out of the battery the other week!) and feeling fresh (no need to overclock the CPU anymore), but there won't be much more I can do with it now as the hardware probably won't support 2.4 so at some point I'll need a new phone. Maybe that phone will be an Incredible S, but only if I can put the latest vanilla Android version on it and it's well supported by good ROM developers.
So, it's definitely not something that everyone would need or even want to do, but it would seem that the majority (perhaps not the vast majority) of those who try it, like it and stick with the approach.
I'd say, 'see how you go'. If 2.4 comes out with a killer feature or 3 that you just must have and you have to wait a long time for HTC to release an update then use the forum to see if there's a custom ROM that suits your needs. Perhaps you won't get to that stage until the middle of next year when another Android release comes out. Or maybe you just get a bit bored of how your phone works/looks. Rather than get a new one, consider rooting and/or flashing.