I had no idea that expansion cards had so many uses as I have essentially one main use for my expansion card slots. I, like @sammyG, am a video editor for a high-end video production company and require the use of both very expensive and highly specialized video expansion cards.
Many PC gamers will spend a few hundred dollars on the latest and greatest video card expansion and some will even add custom cooling and multiple cards to bridge and increase performance. My needs as a professional unfortunately require me to purchase the extremely-high end video workstation expansion cards.
These cards typically range in price from $1000 to $4000 and can often cost more then the computer system itself. The advantage that they add when processing and rendering special effects is well worth the money spent however.
The technology is constantly changing but the ability add onto my system and modify it through expansion cards makes the expense well worth it as my video editing is dependent on the reliable speed and excellent results that they provide.