The number one place where you can wear your semi formal dresses is when you are going out to have a drink. Yes, of course most people wear cocktail dresses to drink cocktails, but there are many other options. I, however, don't recommend that you wear the most formal looking dress to a bar.
Joining a ladies clubs will give you a great excuse to wear your collection of dresses weekly. Most of these organizations require women in the club to only wear skirts and dresses so instead of having to go out and buying some, you will already have a collection of dresses to start off with.
Go to an opera or a museum. Because both of these places are considered classy, you are expected to dress up. A formal dress is very classy and will also make you stand out among the others inside the building.
The great thing about Halloween is that people expect you to be wearing something different compared to what you normally wear. So, going to a Halloween office party wearing one of those semi formal dresses and maybe a masquerade mask over your face is the low cost solution for the perfect costume!
Have you ever been on a cruise? If you have, then you know that there are special occasions during the cruise where a formal outfit is required. This is another great place to wear formal dresses. At most times you will be wearing it to dinner, but also to all the parties.