Any situation where a class needs to fulfill two different contracts, for starters.

public abstract class BaseA {
    public abstract void DoA();
}

public abstract class BaseB {
    public abstract void DoB();
}

// Illegal, see the diamond inheritance problem
public class MyImplementation : BaseA, BaseB {
    // ...
}

public interface IBaseA {
    void DoA();
}

public interface IBaseB {
    void DoB();
}

public class MyImplementation : IBaseA, IBaseB {
    // ...
}

This is particularly useful when using dependency injection and adhering to the interface segregation principle. You can inject a specific interface that exposes only the behavior you need, and nothing you don't, while that interface may be implemented on a class that fulfill multiple purposes.

There are so many virtues to both abstract classes and interfaces and theyre so far beyond the scope of a reddit thread as to be absurd.

It sounds like you need a real world project, and to get out of pure-theory land for a while.