After losing out on Brock Osweiler to the Houston Texans and seeing Peyton Manning retire, the Denver Broncos had to move on to backup options. While Colin Kaepernick remains their top option, the team acquried Mark Sanchez from the Philadelphia Eagles for a conditional 2017 pick.
Sanchez currently sits as the best quarterback on the Broncos roster by far, and the only quarterback to attempt a pass in an NFL game. Trevor Siemian is no longer the only quarterback on the roster, so Sanchez as of now would be the clear starter.
But the Broncos will certainly continue their efforts to try and land Kaepernick, or explore free agent options like Robert Griffin III or Ryan Fitzpatrick. Sanchez likely will not be the Broncos' starting quarterback when the season begins and was just a move made by the franchise to have some stability in a backup who could start if needed for a stretch of games.