• give you a chance to practice your presentation skills

• let you show that you have met the goals of Thesis A
  • convince others that you're studying an important/interesting problem
  • demonstrate that you've done some research/thinking about it already
  • have a plan for the rest of the year to solve the problem

If you already have some results to show, that's a bonus.

Target your seminar at fellow thesis students except target the hard-core technical stuff at your supervisor and assessor.