Slitherine

What do the bonus missions give one

It depends on the mission. On default campaign settings, you're guaranteed another hero for completing it. There may also be commendation point rewards granted for completing optional objectives in the bonus mission that you can spend down the line. If the bonus mission uses your units (nearly all do), then the units you deploy will gain experience (up to the campaign's experience cap) and progress towards medals. Equipment you capture goes into your storage.

And, of course, the fun of playing another mission.

thanks