The ultimate training objective of tai-chi pushing hands is an art of energy redirection, using throws as execution objective like aikido throws. The similarity with aikido is that both have to feel the energy of one's opponent. Whereas aikido focuses on wrist feel, tai-chi is a full body feel. Although the latter looks more "practical", the former does have the benefit of equal training: the uke (the one receiving) is trained as much as the one who executes the throw. Again, in both the practitioner must be relaxed body and mind, in order that he can feel the energy direction of his opponent. The redirection must be smooth and unobstructed, otherwise it will fail. Hence, for successful energy redirection, the throws look spectacular and fluid. Its fluidity just look like one doing the tai-chi Form in the right way.
The following is a demonstration tape by a Hong Kong Wu-style master who is very proficient on this aspect of tai-chi:
Similar to aikido, it begs the question of relevancy to actual combat situation. It is important that one must understand the benefits as well as limitation of every form of practice. Here is a short interview of Bruce Lee of how he looks at tai-chi (he is certainly right on the fluidity side as well as its prevalence among older folks):