If the parent fish are not in good enough condition to spawn, then they may not spawn: for individual fish, part of conditioning may be improving the general health and happiness of the fish so that they will spawn at all.
Spawning in some species is also partially triggered by an increase in high-protein food, which simulates an abundance of food, which is a good time to raise offspring, so some conditioning may be more of a spawning trigger than actual conditioning.
There are many reasons for conditioning, some of which are not even fully understood. As a very general rule of thumb, fish which are kept in ideal conditions are less likely to "need" conditioning (they may need only a spawning trigger or nothing at all) than those which are kept in more "generic" conditions. Since the subject is not particularly well understood, there are a lot of myths circulating and only very few people can say that one or another thing is really fact or not. For the average person, it's probably best to just go with what works for most people.