Part 3 “Explanation” of Form I-765WS is an optional field. It is NOT obligatory to explain your economic need or to submit any supporting documentation.
Meanwhile, if you choose to provide an explanation regarding your current financial information or your economic need for employment authorization, USCIS will take your explanation into consideration. Note that you do not have to be in severe financial hardship to apply for an Employment Authorization Document. For example, among the valid reasons to apply for a work permit can be: to save money for an emergency healthcare fund, to collect money for college, to become self-sufficient, etc.