I religiously keep all my old phones (and other peoples if they don't want them!). Truth is I need them. I decided to get into Android app development as in my spare time, and having as many different phones as possible is really useful. Different screen sizes, different versions of android, some are fast, some not so fast, but all useful.
I currently have: OnePlus 5 (current phone), Nexbit Robin, Nexus 5X, Nexus 5, Nexus 4, Nexus 10, LG Leon and a (very old) ZTE Blade.
...but that means I have Android Oreo (8.1 and 8.0), Nougat (7.1, 7.0), Marshmallow (6.0), Lollipop (5.1 and 5.0) and Kitkat (4.4). This covers testing for 95% of all android phones in use at the moment. Can you believe Nougat and Oreo only account for about 26% of all android users!
It is actually really surprising that one phone can cause a bug that all others don't. Drives you nuts sometimes, so it's better to have as many as possible.