The 25-year-old scored nine goals for the Quakers last season.
Purcell told BBC Radio Northampton: "I knew the set-up here and it looked like a good club, somewhere stable where I can enjoy myself.
"Darlington was my first chance in English football. I made a little name for myself there and hopefully I can make a bigger name for myself here."
Purcell added: "But I'm a part of a team, and I don't expect to start or be the star man.
"I expect to work as hard as everyone else and earn my place, and if that means being on the bench or playing wherever the manager wants, I'll do that."
Meanwhile Cobblers boss Ian Sampson has confirmed Marcus Hall has been offered a deal to join the club.
The 34-year-old left-back, who is currently a free agent, featured in Saturday's friendly with Long Buckby.
Source: BBC Sport
Source: BBC Sport