In many cases it is practical but people wrongly assume it isn't. In cases where its not practical exhausting a subset can still be very powerful.

E.g. for a function that takes two floats (x,y) test every value of x input with y={0,-0,1,nan,inf,-1,x,-x,2x,x/2,FLT_EPSILON,FLT_MIN,FLT_MAX,random,random,random...}, and then repeat with x,y swapped.

A little knowledge of the domain can help pick better special values, but there is a trade-off in introducing the same blind-spots that permitted bugs in the first place. Still: testing just one argument on all values because that's all you can afford is still a lot better than only testing a couple special values for both arguments.

Exhaustive testing, like randomized testing catches errors you weren't even thinking about-- but unlike random testing exhaustion can reliably catch bugs which are extremely rare. "Partial exhaustion" can have some of the same benefits.