Obviously, as you are getting more customers (and bigger customers), your managed IT assets grow, sometimes exponentially. Keeping all stuff in separate Excel sheets might be a problem.
You have to buy software and hardware, keep track of users, you’d want to know what software is installed and where it is, etc.
One simple answer to this is: use a tool that’s been developed for it. Spiceworks is currently considered by many as the most advanced free set of tools for IT outsourcers.