The short answer, which Termite Society stated is correct. A motivation for doing so is to show some sort parallelism or simultaneous actions.
In my opinion, fiction writers should focus on cause and effect as opposed to events taking place at the same time.
Using your sample text:
She asked and scratched her head.
She scratched her head and asked.
Notice in the two versions of cause and effect. She asked and something happened afterwards or she scratched her head and something happened afterwards.
Another reason I choose cause and effect over progressive tense is verb strength.
Stronger: she or he asks. He or she asked
Weaker: she or he is asking. He or she was asking.
As writers, we could seek out a balance where there may be points during a story’s plot where parallelism makes sense or seems plausible, epically in scenes where less showing and more telling may be appropriate like transitioning between plot points and moving time forward.
A plausible example:
He walked, smoking a cigarette.
That’s easy to do for any tobacco user. It happens every day.
But again, I emphasize cause and effect overall.
I also emphasize the evidence of smoking in this example as opposed to progressive tense.
He walked and took in a drag, a warm, airy flavor flowed into his mouth and into his lungs. He forced three short exhales and three smoke-rings floated into the air.
Solid exceptions using the progressive tense:
Character dialog – We use progressive tense naturally when speaking and so should our characters.
Character Internalization – Same as dialog. Internalization is our thoughts and our characters think also.
Similes – Making similarities to show vs. tell.
Hopes this helps.