With those RTs and @ replies sprinkled throughout your stream, it immediately shows that you're an engager. And other engagers are always on the lookout for those who are like them to follow. So you'll be attracting people who, like you, are keen to interact, learn and share their expertise. (That saying "birds of a feather" definitely applies here!)
On the other hand, if you just keep tweeting links to your own stuff or sales pitches, without any engagement, you'll attract others like yourself. Sure, you might get lots of followers, because they see that you've got quite a few followers already, and that you tend to follow back. But all they'll be doing is following you to beef up their follower count, and no other reason. So you'll be tweeting stuff that they never click on, or only rarely, and vice versa. It is pretty close to pointless.
That's why whenever I log into Twitter now and see who's started following me, I immediately look for those all important retweets and replies. If I scroll down to the bottom of their feed an haven't seen any of them, I just don't follow them back. If they have them, however, I usually reciprocate. I suspect that a high proportion of Twitter engagers use these same criteria for whether they follow back as well.