This is necessary in order to prevent last-stable binary secondaries from seeing the FCV document change in the oplog collection BEFORE the primary bumps its internal wire version min and closes incoming connections with last-stable binaries. That would cause the last-stable binary secondary to crash.

For example, if a v3.6 secondary obtains the 4.0 FCV value from the v4.0 binary primary's oplog, it will invariant and crash on seeing a BadValue: it only accepts 3.4 and 3.6 FCV field values.

See the linked BF for further elaboration on HOW the secondary manages to acquire the oplog entry.