Flashing on a roof is a very important part of a proper and professional roof install and serves a very important role of directing water away from critical areas of the roof so that the water does not drain down around chimneys, roof vents and other roof penetrations.
Without the roof flashing the water can get down under the shingles and into the roof penetrations and cause a roof leak.
Any professional roofer will install roof flashing and if they do not then they are not very good or trusting roofers.
And wherever the roof plane meets a vertical surface like a wall or a dormer the flashing is installed to surround roof features, such as vents, chimneys and skylights.
So if you're installing a roof whether shingle or metal and you have vents, chimneys or skylights you need flashing installed to keep the roof from leaking.