When my partner started seeing his doc, I went along. It was his docs wishes. My partner was going once a week; and I was asked to attend once every four weeks. Some times either my partner or the doc would ask me to come in and when they felt they had issues they needed to discuss together, they told me not to come in.
Having the partner attend is apparently a good thing because sometimes the PTSD sufferer won't open up completely. With the partner there, the doc gets a rough idea of what is really going on.
If your partner wants you there, that's a good thing (in my opinion). It means he/she respects you enough to include you in the learning to heal process.
By attending, you also get a chance to speak to the doc about your concerns, which your partner may not be talking about.
Congratulations on not only supporting your partner, but having his support as well.