Food was always present at the defenses I've attended, and always provided by the defender. I assumed that it was a "thank you" move, as Etikate suggests. These same defenses have always been professional, in-depth, productive sessions. I assumed that this was standard practice, since everyone seemed so used to it and to proceed with business, as usual.
One member of my committee was offended by the homemade cookies and brownies I provided, while the rest were perfectly fine. I had no idea that anyone would object, since it seemed to be the norm. I'd also never attended a defense where he was a committee member, but had attended plenty of defenses containing the others. I was taken aback a little, but said, "I apologize for offending you." The others shrugged and ate his portion. :- )
So, yeah -- try to get a good read on individual committee members. In my case, one person's taking offense didn't matter in the greater scheme of things; he was just more difficult to deal with for two hours. But two hours, out of an entire life, ain't nothing. If food simply isn't "done," however, it's important to know that.
On another note:
Baking, for me, relieves stress, and it's also a chance for me to incubate, to invent, to reflect -- if my hands are busy with something else, my brain usually "unclogs," and gets working again. I'd have baked anyway, and provided treats to the office staff, if my committee hadn't wanted them. If I'm coming in prepared, with detailed notes, extra paper and pens for guests, extra copies of my thesis for guests, copies of the video and text under examination, etc., then I'm puzzled as to why it'd be assumed that my mind wasn't focused enough on the work itself?