There are a lot of other voices you can use such as strings, sax, brass, pianos, organs. Your guitarists cannot do those voices.

Your band should play songs that takes advantage of those voices. If they just play songs that only needs guitars and drums then you are obviously not needed. There are many many songs that utilizes other voices than guitars. And in a song, a keyboardist may only play some portions and sometimes more or less but it certainly brings a lot to the songs. Also, you can take turns with the lead fills. Some with guitar, some with sax, some with organ, etc, etc.