Sometimes you need to get two separate applications to play nicely together under the same domain.
If one of these applications is especially greedy with regards to the routes/URLs it wants to control, then it can become a tricky project. Using subdomains can often help, as the subdomain can explicitly address the other application. However, if you need content from both applications under the same domain, and you don’t have access to or the desire to fumble with the routing (e.g. you use Heroku) then a different approach might be required.