My iPad1 replaced my Mac Book, I just didn’t use it anymore. Definitely revolutionised the way I work/play as it where, very portable, instant on, great battery life. I have a Mac Mini as my desktop and an iPhone.
As per above it’s used for email, web browsing, document reading and mark-up (for work), lots of useful and fun aps, great for travel especially on flights – having my own movies, online videos, Skype etc. Yes you can do the same things on the phone but the screen is too small on the phone really.
OP you might like to consider an Apple TV – works great with iPhone and Mac’s – only £100 and perhaps don’t get the iPad if you’re not sure the other half will use it.