The intention, I believed, is to not break existing game builds. If the engine itself is updated, there is a very high likelihood that your game builds will break on the newer engines, potentially even completely rendering the source code useless in worst case scenarios. Thus, if the engine versions were installed separately, such as UE_4.XX, or even UE_5.YY, then everything will be intact. Your game works, and there won’t be any issues with engine version migrations.
However, the downside is it takes up a lot of hard drive space. Especially with SSDs on (almost common) portable computers / 2-in-1 laplets (laptop/tablet) / Surface Pros.