One good reason is because not everyone uses Linux, so being able to scrub out malware in a relatively secure environment is a good practice. Linux powers many file and web servers that Windows and Macintosh computers connect to. I know that I wouldn't want to find out from an irate client that I gave them some malware! While Linux itself is relatively secure (if you patch it religiously and practice good basic security protocols), there have been some Linux exploits. (The word "virus" is hopelessly out of date.) I'd rather have ClamAV scan my Linux machines' filesystems from time to time just in case.