Why valleys matter
Valleys concentrate the runoff of two roof planes into one channel, so even a modest debris dam moves a lot of water somewhere it should not go. Wet debris sitting on shingles also holds moisture against them for months, which is how a 25-year roof becomes an 18-year roof.
What clearing involves
Working the valleys and the roof surface near the eaves with a brush or blower, from the roof or a properly footed ladder, before the gutters are scooped. Doing it in the other order just refills the gutters. It adds minutes to a job, not hours, which is why a good crew includes reachable valleys by default.
A word on roof walking
Tile roofs crack under foot traffic and steep or wet roofs put people in the ER. A responsible crew will say no to walking some roofs and work from ladders or with extension tools instead. Treat that "no" as a good sign about everything else they do.