会 has many meanings
in this context, "会" functions as "would"
Would: modal verb (INTENTION/ POSSIBILITY)
"为什么我(会)觉得历史课很无聊?" = "why (would) I think history class is boring?"
If there's no explanation following the question, it is a rhetorical question
Example of 'would' as a modal verb :
"If it doesn't rain, I would go" (INTENTION)
"One of them would die if they fight each other" (POSSIBILITY)