The brass family is arguably one of the oldest sections of the orchestra, and has been used for memorable moments in music for centuries.
Before the advent of valves, both piston and rotary, brass players would be limited to playing the harmonic series in a particularly key, and a brass performer would carry a series of tubes of varying length, so they could play in more than one key. With the invention of valves, players were free of this manual change process, and composers were quick to write new music for instruments that handle chromatic changes with ease.
The trombone is a reminder of days gone by, as it's slide mechanism has been unchanged for some time, and the principal remains sound to this day.
French horn players were the most appreciative of the addition of rotary valves for their 21 feet of tubing, and the modern French Horn comes in variations that range from a straight through horn, with no valves, to complex instruments possessing 4 valves, and key change triggers, the most obvious example being the F/Bb French Horn. In the evolution of these instruments, there are many changes, including piston valve trombones, and multiple trigger trumpets, but in the modern orchestra, as a matter of normal use, Trumpets, French Horns, and Tubas use piston or rotary valves, and the trombone uses a slide.