You don’t “need” either. They (TSA Pre and Global Entry) are both programs to make things go faster/easier.
Global Entry comes with PreCheck btw as its number can be used for both.
I have Nexus which comes with Global Entry privileges and used it for the first time last year on my return from Europe and it was pretty slick, and made entry back into the US painless but ultimately it didn’t save me any time as I had a checked bag and thus had to wait for my bag and by the time that showed up everyone had cleared entry even in the standard entry lanes. (They are visible from the Bagage area at the airport I came into)
So while NICE Global entry works best in either locations that are completely swamped or if you are flying carry on only. But I always seam to have a checked bag on my return from overseas. (Too many souvenirs perhaps?)
Personally I find PreCheck the more useful program/benefit (yes you get this with G.E.) as it’s avoids the long lines at security.

Now if the US and Europe /GB ever get their act together and make all the programs interconnected so that you can use GE to say get into Germany then that would be a differ story,

Still if you have the time and money or a way to get GE reimbursed then you may as well pick it up or if you travel exclusively with a carry on. Then it is useful.
In my case my local airport (that I almost exclusively return through) is pretty good with its entry so it does not take very long. Last year returning with friends (who do not have G.E,) was about the longest I have had to wait ( 15-20 min) that was still shorter then the time it took our bags to get unloaded (but not much) so even in that case it would mot have saved any time for us. And while not the last of our plane to clear boarder control we were close as we had stopped and used the restrooms.

So it is a matter of personal choice and how one travels that determines how useful GE really is. But PreCheck is alwas worth it if just so you get the less strict rules.