Getting around in an attic is sometimes made difficult by head bangers, there for uncertain reasons. The function of ties under a roof peak, between roof joists, is to resist uplift and ruin of a roof, in storm conditions. Some will think the purpose of ties is as found in code requirement of collar ties not more than one third distant from tops of walls, to the roof peak, to hold walls in absence of ceiling joists. Collar ties will have little relevance to residential construction.
Here is a slide show review of the problem, and some solutions.
Here is another view of the solutions, in a web page.