Have a look at the 2014 DRAFT v1, Tor Design Document (which seems to be more up-to-date than the version in the Tor SVN repository).
Cells are discussed in section 4.1 (page 5).
The short answer is: varying the length of cells makes detecting Tor traffic more difficult. You can't just keep an eye out for cells of 512 bytes.
Fixed-size cells provide some resistance to traffic analysis but are
inefficient, so some control cells are variable length, where the
ability of an attacker to detect their presence doesn’t affect
security. Fixed-size cells also make the
packet-size distribution of Tor distinctive, contrary to the goal
of protocol-fingerprinting resistance. Therefore a variable length
padding cell was introduced (but is currently unused)
to allow the implementation of schemes to disguise packet
Note: I'm unsure if the variable-length padding cell, mentioned in the final sentence, has actually been implemented since this document was created.