Can the Jobo 3010 not take 5x7 film? It says 10 4x5s?
What's the point of the sponge?
The 3010 is for 4x5 film
Absorbing water in between rounds of processing.
The 3010 has five wells, and each well has ribs in it to allow two sheets of 4x5 to be inserted securely without slipping over each other during the processing run. It should be possible to fit a single 5x7 sheet in each well, but I don't know whether the ribs might result in uneven development. Although I have a 3010, I've never tried it with 5x7. The 3006 has six wells without ribs, and is designed to be used with either 4x5 or 5x7.
As Vinny said, the sponge is to help in drying out the wells if you want to use the drum immediately for another processing run.