There are several reasons you might want to download Windows 98. For example, if you need to install Windows 98 but you've lost your original Windows 98 Setup CD, downloading Windows 95 would get you out of that bind.
You might also might want to download Windows 98 just to try out the operating system or to put it on a second computer at home. Windows can be expensive so finding a place to download an older operating system like Windows 98 for free is an enticing thought.
Pretty much everything is available on the Internet for free, right?
No matter where you find it, any Windows 98 download you find online is illegal. The only legal course of action when you need a copy of Windows 98 is to actually buy a new copy of Windows 98. It's that simple.
It's important to note as well that even if you did download Windows 98, what you would get is an image of the Windows 98 Setup CD. For example, you would probably download an ISO file like windows-98-se.iso, or something like that. You would then burn that ISO image to a CD that you would then use to install Windows 98.
In many ways, however, what you actually pay for when you legally purchase a copy of Windows 98 is the product key (sometimes referred to as a CD key or key code, or incorrectly as the serial number). This unique number is required during an installation of Windows 98. So even if you did download Windows 98, you would still need a valid Windows 98 product key to install and use Windows.
Note: If you actually do have a Windows 98 CD but you're looking for your product key, there is a way to find it. See How to Find the Windows 98 Product Key for easy instructions.
If you do in fact have your Windows 98 product key but you're missing your Setup CD, you could argue that, since you did actually purchase Windows 98 and you have a valid product key, downloading a Windows 98 CD image from anywhere should be within your rights.
At this time, however, the only legal way of obtaining a Windows 98 CD is from from a legal purchase of the operating system. I would suggest contacting Microsoft for a replacement Windows 98 Setup CD, assuming you can show proof of purchase. However, since Windows 98 is a retired operating system, you have trouble doing that.