Whether or not you need sewer backup insurance depends on the type of apartment and the level of risk it brings. For instance, if you live on the tenth floor of a condo building, you likely won't need sewer backup insurance. If you live in a basement near water, on the other hand, sewer backup may be a top priority.
However, know that if there is a clog in the drain below your unit, all the flushing from the units above yours could spill out through your bathtub and sink drains. Though rare, this is a situation in which sewer backup insurance could come in handy for an apartment unit.