Checking that your client can read & write assists both you and them. Attempting to use questionnaires, journals or bibliotherapy with someone hiding their illiteracy could end your intervention before it has begun.
Literacy isn’t essential in therapy: psychological therapies aren’t called “talking cures” for nothing and physical therapies usually require little in the way of reading or writing. Clients who cannot write can keep pictorial records or use voice recorders (now built into many mobile phones) to keep notes of thoughts or actions. Much of the literature therapists would wish to hand out to clients could, with a little effort, be offered as graphics or video & audio recordings.
The difficulty for most therapists will be in identifying clients whose illiteracy may be one of their most closely guarded and shameful secrets. Ticking the boxes on your questionnaire need not mean that the questions have been read. Phrases like “your writing is too small” or “I’ve left my glasses at home” may mean exactly what they say, or may be well-practiced cover-up routines. Allocating an illiterate person to bibliotherapy can be a waste of their time and yours.