Latches

If you do not assign an output for all possible conditions in an if or case statement (that is, incomplete assignment), the software infers a latch. Trion®, Topaz, and Titanium FPGAs do not support latches natively in hardware. The Efinity® synthesis tool infers look-up tables (LUTs) to provide latch behaviour.

Because latches are turned into LUTs, they can use up resources you could be using for something else. So you want to avoid them when possible.