THIS is why scarifying and aerating are SO important!
It's JUST grass... isn't it?
Well, err yeah.. it is just grass.. but with grasses whose nature is to spread and thicken (many grasses), eventually things can get too thick.. but also, grasses build up a layer of thatch (a matted dead grass type of layer down on top of the soil, but nestled in between the grass blades) which slows down how well water, fertiliser and even air can get to the soil.
And, well it's kinda important really!
What will happen is the grass slowly loses its strength and resilience. It's not able to be play ball at full strength and so it becomes weaker.. Disease can set in, and also weeds.
So look at scarifying and aerating your lawn once a year.