I don't have much sense when it comes to building things, but I can see the need for a trimmer now. For all my life, I have never once considered what keeps a house from collapsing over windows and doors. Now that I think about it, certain types of openings would need extra strong trimmers.
If you had those big bay windows that cover half a wall and sometimes extend almost all the way to the floor, you would really need to provide extra support around the frame. This could be a really vulnerable area when strong winds whip through the yard.
The same thing would apply to sliding glass doors. They cover such a wide area, and a trimmer would help support the weight of the house over the glass.