Roll call votes are used in several different circumstances, always to clarify the vote for the record.
Someone challenges the chairs decision as to the vote count. “Excuse me, Mr. Chair, but I hear four voices in support, not three and I think the vote was 4-3, not 3-4. Could we please have a roll call vote?”
You are taping the meeting as the only means of keeping minutes. A show of hands is not going to give the secretary the info needed to transcribe the minutes and indicate exactly how each person voted.
The board is very large and it is difficult to track how each person voted with a fast show of hands or a voice vote.
Some large boards have a practice of always performing roll call votes because it may be hard to look at seven hands at once, or distinguish seven different voices.
Roll call is used for clarity. Remember that in Oregon, the audience must be able to tell how each person voted, and the minutes must show how each person voted.