Ireland constantly churns the OL. Thomas is a replacement for Incognito to lower salary and give Philbin a young lineman with which to build. In then short term he may compete for Jerry's spot if Lance Louis is not quite healthy.
If everything works out, Ireland will cut ties with Incognito either this off season or next. Thomas is also an emergency tackle if we have injuries.
I'm not as high on Thomas as most seem to be.