You need one of two things in place to sell any item: the clients either want or need your product line or a combination of both. Without a want or need from the client, you cannot sell much of anything to anyone. So, the advantages of share shifting over getting new customers/clients are:
The want and need are pre-established
The client is already buying, so spending habits are historic, thus forecasting can be done for pre-qualifying their future spending consumption
Having purchased from your competitor in the past, selling strategies on your product line's advantages over your competition can be highlighted and exploited
A pre-qualified database for quicker sales effectiveness, ramp up/your "low hanging fruit"
Builds revenues in a soft or downward trending marketplace
No wasted time on "uncovering" needs in your sales stage progression. Due to this, you can move the client quicker through the sales pipeline to the purchase phase
With share-shift marketing, your top-line growth will be faster than with most other sales campaigns, due to the fact that you have an accelerated cycle built into this sales effort. So, go on out there and get a bigger piece of that market share pie, before someone else does and then throws it in your face.