As has already been stated, you need a specific type of 98SE disk to be able to upgrade. (Not sure why this was thought of a s a good idea, but who knows what Microsoft thinks?)
I think the point of asking "why" was because you are upgrading a 7 year old operating system to a a slightly more recent but still 7 year old operating system.
Sure it'll still work as well as it did last century, but on a modernish pc (say one less than 5 or so years old) XP or 2K would probably be more suitable, certainly more stable.
There's nothing wrong with 98/98SE, but equally if you have the opportunity to upgrade to something more recent, go for it.
(As far as I'm aware the only major differences between 98/98SE are trivial except for the usb support, so I'm not sure exactly what upgrading will gain you).
Equally it is becoming somewhat difficult to get drivers for any recent devices (printers, scanners, cameras) that work under 98.
Having said that, if you have some rare/old bit of kit (perhaps an elderly, but high-end printer that works perfectly) you might find you can't get it to work under XP or something more recent.
If your reasons to upgrade are mainly kiddie-focused (perhaps an old game that insists on 98SE?) XP can handle almost all of them. Some require a bit of fiddling, but XP has a compatibility mode that allows it to support games not written for it, but its predecessors.
Good luck with it either way.