The only thing UAL needs from JB are the gates in JFK and BOS.

They don't need the planes (yet), staff, or pax

I see a codeshare or some gate agreement thing to get UA premium passengers more NYC/BOS options. Especially on the northeast to FL shuttle.

I think they are just doing some partnership thing to prevent someone else (Alaska) from partnering and doing something similar or buying them.

JB needs a partner. Same as F9 and Spirit.