Why are some POI's needed?

No, you've pretty much got it.

The POI's preloaded can be out of date so custom POI's allow you to have more current info. Plus on many units, the custom POI's will show up when zoomed out further than the built in ones do.

For example, on my Zumo, the built in POI's don't show up until I'm zoomed in to 300 feet, custom POI's show up at 0.3 miles and Favorites will show up at 5 miles (but you are limited to 500 of them).

The other use is that many things that people interesed in aren't included in the builit-in POI's.

And lastly (at least for Garmin units) you can set speed and proximity alerts for custom POI's. You can't do that with the built in ones.
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Plus, the POIs as preloaded don't necessarily include the things YOU happen to be interested in. 6 million POIs sounds like a lot, but that is barely a beginning as compared to the number of points that could be of interest to someone.

Which I think is its best use. I set up POI locations for well known speed traps, then I attached a speed.

Now when I as I approach these speed traps I hear a single alert, (to notify me its ahead) then if I'm going faster than X mph, my Garmin Unit will Ding, till I slow down.

TO be honest all the speed traps that I've known from being a kid I still remeber before I get there, but I also set custom POIs w/ speed alerts on the highway were state trooper locations are (which if I would have seen the sign I would have slowed down but if I would have missed it on the HWY I probably wouldn't have slowed down) so here it has been very helpful as well.

My most useful POI set is one that I made up of the ATMs for my bank. That way if I need to pick up some cash, I can find one along my route easily and avoid the extra ATM charges from using another bank's ATM.

In many cases I prefer the custom POI's as I have them entered in subcategories. For example, when I am looking for local Farmer's Markets, I just look under my Texas Farmers Market POI's to find the nearest ones. The same goes for state parks, rest areas, etc.

I'm still struggling to figure out the best use for the built-in ones. I could see wanting to use them if I'm in a strange place looking for a restaurant or bank, but I can never quite seem to find what I'm looking for if I'm in my own town and know exactly which store I want to go to.

Well, since I travel a lot to new places that I am not familiar with, POIs are great to have!!! I probably wouln't have gotten the GPS in the first place if I didn't travel extensively.

Especially searches for attractions, etc. I find places I didn't know about that are driving distance from my home. Look through the categories and do a search. Sometimes it just reminds you of places you knew about but haven't been to in a while.

That is so true. VA is full of history (which I like) and you just forget sometimes that they (the attractions) are there.

The easiest way is to make the locations (your favrite POIs) is to punch in the address and save - or get to the location and hold the car or whatever symbol you use - it should pop up and save. If you want to add a telephone number you have to manually save it and if you hold the symbol you will have to rename it but you can do this quickly.
You can have up to 500 favorites so you will have to live in a big town or a be a traveling salesman to go over this number.
I have used a GPS for the last seven years using these features - very rarely used the internal POIs except when I am lost, download only a few of the customs and life has been good.