Thru axles keep the wheel in alignment better and this is important with disc brakes where the tolerances are much tighter than with rim brakes. The Boost hubs and axles are much stronger and so used with e-bikes and mountain bikes where the support for the fork is important. The thru axle also allows for a lighter fork to be used as the thru axle connection is a much stronger one and better at dealing with lateral stress on the wheels.