Most computer applications now days tend to create very large files. Most people use their computers to create and share multimedia including: images, videos, presentations, and applications.
Many of the files created by multimedia applications can easily exceed hundreds of megabytes.
Since email servers are usually shared there is a fairly small amount of disk space and there are policies in place to restrict the size of attachments that can be sent.
Gmail for example only allows for attachments up to 25mb in size, how much exactly is 25mb?