Mine was for a range of reasons.
I have an iMac, iPhone and two windows laptops however my iPad is used perhaps just as much as my primary computer which is the iMac.
For my business I used to print work orders, back up check lists and other paper work on which I would fill in the details by hand and get the customer to sign the completed work order. These were scanned and filed. Now all my customised PDF files can be filled in on my iPad, the customer can sign directly within PDF Expert and it can all me synced via Dropbox to all my other machines.
Commitments mean that I have to travel to Germany once very few months I used to take a laptop but my iPad comes with me. It does everything I need such as grabbing my email, web browsing, Skype and also keeps me entertained with videos, books and games while travelling.
It's incredibly light with a great battery life and when kept in a case can withstand the bumps and knocks of being in my bag.
Lastly it's fantastic for reading, browsing and watching video in bed or around the house without the heat usually generated by a laptop. I open the case and it's on instantly and ready to use.
I wasn't entirely sure how or if I would use the iPad and how it would fit in with my current gear however it's proved to be versatile.