You only need one tool to run your business. You can use Excel for bookkeeping, inventory, salaries, invoices, project management, budgeting, presentations, planning, forecast, brainstorming, timetable, to-do list and so on. If you would use a separate app for each of these you would go insane. Easier to learn one powerful tool than hundred small ones.

Take a look a talk by Joel Spolsky called You Suck at Excel. It's on YouTube. It will give you a quick glimpse into how Excel powerusers are using it and how fast you can create something useful.