Business software customers already have a lot of people using a lot of other tools. If you want to sell some software to a business, it needs to integrate with those existing tools. Selling a customer service issue tracker to a software house that doesn't integrate with their source control is a non-starter. Selling a timesheet solution that doesn't integrate with HR databases and invoicing software won't get you anywhere.
Mokalus of Borg
PS - Selling directly to consumers, on the other hand, makes "integration" a dirty word.
PPS - Go figure.