I've mentioned before that I journal about once every three years. But even so, I think journals can give you inspiration in a few ways.
First, go back and read what you've written. I do this more with my writing notebooks than my journals (but then this wouldn't be posted until November 26th when I do "W" in the alphabet!). Some days I'm just full of ideas and they kind of just pour out in a jumble of notes, phrases, words, doodles, food stains, and quotes that I've made up for famous people ("Eggs do the body good...pass it on" -Betsy Ross). I don't usually get any real writing done on these days. My brain is just too scattered. So, I have to go back later and try to sort through the detritus to find the good ideas and be inspired by them.
Second, sometimes the act of journaling can provide the inspiration. There are times in our lives when our emotional baggage can get in the way of inspiration and creativity. Putting your pen to paper and working through those things can open you back up to inspiration. Unclog your brain and your heart and let creativity flow!