From language, you are using (client, assignment) I'm assuming that you are working for a consultancy, agency, or outsourcing company. And this is the perfectly ok situation, people are usually expected to be T-shaped professionals - really good in something, and average and below average in everything else.
I'll give you a concrete example:
I could hire you as a software engineer, for a project starting in 2 months, legal team could then inform that they need another 4-6 week to negotiate contracts and I have no immediate work for you to do. But I have this R'n'D project where I need a User Experience professional. I would feel no hesitation, and will give you a task to design something, even if you don't have commercial experience or eduaction.
If you don't like the nature of the task do communicate it, but I don't think there is any conspiracy, if anything - your manager might be worried that you are getting bored and will be looking for another job and giving you tasks.
Also, consultants regardless of their primary speciality should be well-rounded people ideally, understanding how to write documents, presentations, public speaking, and have some knowledge about design, development, marketing, and sales - so that they can understand requirements and motivations better.