Freeing up the primary is a perfectly valid reason, but FYI, the current belief is that racking to secondary is actually detrimental to beer clarity. The reason is that while the beer is sitting in primary, yeast and sediment are slowly precipitating to the bottom. The top portion of the beer will be considerably clearer than the lower portion. When you rack the beer to another vessel, you're mixing all those particulates back into uniform solution, so they have to start precipitating from the top all over again.
The presence of absence of a yeast cake on the bottom of the fermenter is irrelevant to the ability of the beer to precipitate out any remaining sediment.
Summary: It actually makes the beer less clear, and risks oxidation to boot.