There have already been a lot of points by other commenters, one point that I'd like to add:
It allows to solve "problems of scale" with technical solutions rather than process solutions.
For example: Let's say there's application app-A and app-B, and they decide they want a library lib-1 that they can share.
If they are in separate repositories, this means multiple pull-requests, it means separate pipelines where the pipeline of lib-1 likely won't include the tests of the applications, it means there will be pull-requests to the library which won't immediately be integrated into the applications so that some poor sod has to take care of breaking changes down the road, etc.
If they are in a monorepo, each application can set up tests that need to be fine with the library-code "as is", so any change to the library needs to work against existing application-tests. The price one pays however are pipelines that perform well - nobody wants to wait hours to get pipeline-feedback and such. A monorepo also allows ambitious folks to shine - it's easy to touch many things at once, or to touch things used by everyone, on a pull-request with high visibility, which is an easy platform to get "street creds" as an ambitious engineer.
It allows to solve "problems of scale" with technical solutions rather than process solutions.
For example: Let's say there's application app-A and app-B, and they decide they want a library lib-1 that they can share.
If they are in separate repositories, this means multiple pull-requests, it means separate pipelines where the pipeline of lib-1 likely won't include the tests of the applications, it means there will be pull-requests to the library which won't immediately be integrated into the applications so that some poor sod has to take care of breaking changes down the road, etc.
If they are in a monorepo, each application can set up tests that need to be fine with the library-code "as is", so any change to the library needs to work against existing application-tests. The price one pays however are pipelines that perform well - nobody wants to wait hours to get pipeline-feedback and such. A monorepo also allows ambitious folks to shine - it's easy to touch many things at once, or to touch things used by everyone, on a pull-request with high visibility, which is an easy platform to get "street creds" as an ambitious engineer.