The keefex ship is actually packed with moments and a lot of people believe that if the two were to get together they could help each other's - ah - sanity, if you will since Keefe (especially more towards Legacy) has gotten to be more understanding of Dex and Dex is an incredibly loyal and kind person - someone I think we can all agree Keefe needs.
As for the kam ship, the 'enemies to lovers' trope is extraordinarily popular (it gained this popularity in part due to a Disney show, actually) and nearly the same thing can be theorized about the two if they were to get together. Keefe could understand Tam and his pain (apologies - I'm not sure what other word to use) and vise-versa.
The Fitz/Keefe ship (I am unaware of the name) I believe is popular due to the fact that many treasure the idea of two best friends secretly pining over one another before confessing - much like many imagine with the sokeefe ship ;).
As for Linh and Biana, I'm not entirely sure why some people ship them with Keefe - but, with that said, the only person who reallly knows where the books are headed is Shannon Messenger.
Now, I, personally, don't think Sophie and Keefe would be good for each other (hence why I don't really agree with the ship), but I don't plan on ship-bashing anytime soon.
Please try not to do so unless it is a ship like sophorkle (Sophie and... Forkle. Yeah... pretty gross).
(And I apologize for any spelling errors it's currently 1:40 in the morning and I've been working on things for the whole day).