Speed/Adaptation is an important part of infantry tactics. Strykers and other "light" vehicles have a real impact on battlefield.
=> Infantry rapid moves and manoeuvers
=> As Sgt Joch said, their core target is counter and urban reactivity.
=> But also, they are important for troop morale: knowing that having an Abrams at your side is reassuring, to have a Stryker to quickly being extracted from a conflict zone is probably encouraging soldiers and helps keep calm.
If I may say a metaphor, if we perceived the Abrams as a regular armed force (rather slow but very often ruthlessly efficient), the Stryker could be seen as a guerilla's tool (fast, low fire intensity, but able to quickly move the avenues of approach).
I'm sorry if my english is not perfect, but I'm already working to make myself understood at this time . Thank you for your patience
Damien