Is it better to use categories in urls to group pages?

When I check my competitor, they have all their urls flat. For example,

/service1

/service2

/service3

Even if they are grouped on the navbar with a dropdown under "services".

Is it better to group them to be like:

/Service/service1

/Service/service2

etc?

The thing is there would not be a link to /Service even if it might exist. I'm on WordPress btw.