The reason is very simple because it makes it harder for viruses if the user that is used on a daily basis only has limited permissions a potential virus can do less harm. It simply is safer.
Virus won't be any less dangerous if you are using a limited user account, since if they came with something you installed that already took full permissions and won't run without, they will get to you regardless of the permission level.
The only reason for having a limited user account, is if you share your PC with other users, so you want to be sure whatever they are doing in their own user profile, won't affect yours.
However, if you are still convinced a limiter user account is a good idea, you can just follow the suggestion that has been posted elsewhere: granting you full permission only while installing and activating GSX, then go back to limited permissions.
There's a post on the forum titled "How to Get GSX Pro Working for MSFS 2020 in Non-admin Account"
https://www.fsdreamteam.com/forum/index.php/topic,28730.msg187338.html#msg187338
