Hard disks fail. There can be software problems caused later on by a Win 95 install program. Logic says you need a rescue disk in case the Win 95 boot fails. To do this, the attributes of the file have to be changed. It doesn't hurt to do this operation.
Nothing happens if it turns out the operation was unnecessary.
The command rewrites the master boot record to its default. Most Win 95 installs write over the master boot record on the hard disk, making Win 95 the favored operating system. After these one-time setups, a cold boot will start up on a config.sys menu. There are examples in the DOS help program which show how to set up a menu which triggers after so many seconds to a default item on the menu. That item might be DOS instead of Windows.