There's actually a bunch of reasons as to why Dragapult is a good pick in ou
The first reason being its speed. It's literally the second fastest Pokemon in ou atm. This alone is already a very good reason to use it as unlike the fastest mon, Regieleki, Dragapult has a colorful movepool
Of course, speed isn't the only factor otherwise Regieleki would be number one in usage. The second reason is its typing. Ghost is a very potent offensive typing because only two types shrug it off while being a good defensive type as well. Its typing of dragon and ghost also allows Dragapult to be one of the few safe switch ins to Cinderace, which is always appreciated
The third reason is its access to you turn, which when combined with its naturally high speed, acts as a very powerful momentum grabbing Pokemon given how potent its typing is
Lastly and probably the most important reason is its ability, infiltrator. At the moment, Dragapult is probably the single best offensive check to non scarf Spectrier. With how insane that horse is, anything that can kill it will be used and a Pokemon that can force switches which it can abuse with you turn only adds to the reasons it is used
These are just some of the reasons why it's a good pick. With Magearna being nearly omnipresent in ou, it is almost never gonna run a physical set. It usually runs a specs that allows it to you turn on forced switches or make its special attacks hit decently hard. Blissey and Tyranitar are probably the only ones that can take any special attacks Dragapult throws out but they greatly dislike being used as you turn bait
