Primes are predictable and dependable--you always know where to stand, and what your framing will be with a prime lens. You know "how far away to stand" with a prime lens after using it for a while. After a good period of use with a prime lens, you can mentally envision what picture it will make...you get a feel for its depth of field, you learn how it renders its backgrounds (defocus, background blur, bokeh) and you learn what kind of apparent expansion of space (ultra-wides, wide-angles) and compression of space and background enlargement (telephotos) each prime lens will impart to the images shot with it.
When assembling a prime lens kit, it's often useful to think in terms of doubling/halving your focal length, such as 24/50/100/200. In my opinion, which includes a lot of years using only primes for most things, a few lengths come in really,really hand, and they are the 300mm f/4, a 200/4 or 200/2.8 or a 180mm, a fast 135mm either f/2.8 or f/2, and a 100 or 105mm lens of f/2.8 or faster, and the 85mm 1.8 or 1.4.
Out of those above, the 85mm/1.8 and the 300mm f/4 are probably the single most-useful, most essential prime lenses on either FF or 1.5x bodies. Both the 85mm and 300mm primes give a LOT of very useful capability to one's overall kit...they have very high optical quality, and the 85 is good in low light indoors, usually focuses very rapidly, and can be used for sports, portraiture,landscapes,and travel. The 300mm/4 is invariably a high-quality lens, of decent aperture speed, is usually fast-focusimg, and can be used for sports,nature,landscapes, outdoor people photography, and with an extension tube added, can be pressed into service as a bug-butterfly- pseudo macro lens.