Little or no customization is required. That's one reason why ASPs are popular with dotcom companies eager to introduce their products to the world. There are no up front capital costs because the buyer does not need to purchase hardware or assemble a staff. Many find the concept of renting versus buying attractive. The on- going costs are lower, too, since the supplier can amortize its costs across many customers.
Pricing becomes predictable and simple.
Using an ASP also makes sense in a business environment where mergers and acquisitions are rampant. Switching costs can be lower. This allows all users to take advantages of best practices. ASPs also address the IT skill shortage. High staff turnover and escalating salaries are creating stiff competition for the talented few.