Adding a wide receiver became obvious for the San Francisco 49ers when they attempted to trade for Jauan Jennings.

The need for one grew after their Week 9 win against the Las Vegas Raiders. They couldn't continue to trot out Brandin Cooks and Jacob Cowing.

That led the 49ers to trade for Indianapolis Colts veteran receiver Keenan Allen. His addition isn't anywhere close to a significant move, but he will contribute nicely.

However, it's unclear why the 49ers wanted to trade for Allen over anyone else, like Josh Downs. On Wednesday, Kyle Shanahan shared why they traded for Allen and how he fits.

How Allen fits the 49ers' offense

"Trying to get the best receiver you can," Shanahan said. "It always starts with separating, catching the football. He's always been as good as anyone at separating. Always had as good hands as anyone, and then how good he plays without the ball. He's a guy who blocks. He never takes a down off and is extremely competitive. A very smart, aware football player."

The keyword in Shanahan's explanation is "separating." Despite being 34 years old, Allen is still a fairly sharp route runner. After Mike Evans, he's probably the second-best on the 49ers at it.

The 49ers needed someone who could carve out space for Brock Purdy to throw to. They couldn't rely on him to make tight-window throws throughout the game.

Purdy can make some of those throws, but it's not what the 49ers want to do. It isn't a sustainable formula for success. It becomes more likely for drives to get stalled out or, worse, an interception is created.

Allen won't be a downfield threat. That's never been his game, but he will be proficient around 10 yards beyond the line of scrimmage.

Whenever the 49ers need a quick outlet or a short-yardage conversion, they can look to Allen. Purdy now has George Kittle, Christian McCaffrey, and Allen has his trio until Evans returns.

The fit makes a lot of sense for the 49ers. Having Downs would've been nice to stretch the field, but having a player who can separate almost at will fits the 49ers better.

"We were really excited to get him. We think he makes us a lot better," Shanahan said. "Keenan has obviously had an unbelievable career. It was cool to turn on his tape and watch him in Indy and still see the same guy, so we were pumped to get him.