For example, if they have used Flash, frames, multimedia, or database generated web sites your web site may be invisible to search engines or not index fully in the search engines, making it harder for you to attract visitors.
If they used custom programming in formats such as php, asp, perl, cold fusion or other systems, you will need to know this so that when you move to another hosting company, you can ensure they support the same formats. Cold Fusion, in particular, is a database program that requires special, more expensive hosting and programming.