Why have both a javascript and ruby application

Hello all,

I was brought on to a new application and am trying to learn the structure of everything. When looking at what we have deployed on heroku, there are two separate app apis that are titled the same, but one has spa after it. When looking at the repository, it seems the first one is Ruby and the second with "spa" is javascript. I'm having a hard time figuring out what the point of having both is. When I open both apps they have the same link and work the same, but they are separate on heroku and are separate repositories. What is the purpose of the spa app?