Why notification/request marker interfaces for Mediator?

Whilst I am a massive fan of MediatR, it feels strange to take it as a dependency on all my projects (even those that dont "dispatch" and are merely posting POCOs reference the marker interfaces).

What is the need therefore for marker interfaces in the first place, when the below code works with plain POCOs?

Is there any issues with removing the marker interfaces from Notification and Request?

Handler interfaces

public interface INotificationHandler<in TNotification> where TNotification : class
{
    Task Handle(TNotification notification, CancellationToken cancellationToken);
}

public interface IRequestHandler<in TRequest, TResponse> where TRequest : class
{
    Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken);
}

Plain POCO without marker interfaces

class TestNotification
{
    public int UserId { get; set; }
    public int Age { get; set; }
}

class TestRequest
{
    public int UserId { get; set; }
}

Handlers

class TestRequestHandler : IRequestHandler<TestRequest, int>
{
    public Task<int> Handle(TestRequest request, CancellationToken cancellationToken)
    {
        return Task.FromResult(request.UserId);
    }
}

class TestNotificationHandler : INotificationHandler<TestNotification>
{
    public Task Handle(TestNotification notification, CancellationToken cancellationToken)
    {
        Console.WriteLine("hello");
        return Task.CompletedTask;
    }
}

Main

static void Main(string[] args)
{
    _ = new TestNotificationHandler()
                .Handle(new TestNotification(), CancellationToken.None);

    var result = new TestRequestHandler()
                        .Handle(new TestRequest() { UserId = 111 }, CancellationToken.None)
                        .Result;

    Console.WriteLine($"result is {result}");
}

C# fiddle

https://dotnetfiddle.net/HTkfh9