They mark the beginning and end of the boot section of the configuration. If you add any boot commands, this is where they go. For example, if you want to boot to a different IOS on the flash, you would need to add a command to do that and this is where you'd see it. I'm not even sure you can remove them, but there is no point in doing so.
You can't remove those commands. If there is nothing between the two commands then you know you are booting your IOS from flash. In my opinion, its useful to use the boot commands when you are upgrading your IOS and want to keep the old IOS on the flash. You could specify in a boot statement to boot to the new IOS instead of the old one. You could also have multiple boot statements for instance if one IOS isnt on the flash or something go to the next IOS (the first command takes precedence). Keep in mind though, if you use the boot statements be sure it is exact as to the location of the IOS. Here is an example
boot system disk0:s72033-adventerprisek9_wan-mz.122-33.SXI10.bin
boot system disk0:s72033-adventerprisek9_wan-mz.122-33.SXI2a.bin