I use a refractometer to monitor the boil as well as to tell how effective my sparging of brew in a bag grain (ie when to stop sparging).
If it looks like I will be under for the starting gravity I either bump up the boil or in one rare case add DME to compensate. Reason is
its quick easy and only requires a small sample. small sample particularly good during boil because no cooling required.
Disadvantage is that even with no alcohol content, a refractometer is not as
accurate as hydrometer. Refractometers are only ~100% accurate for gravity measurement with pure sucrose. Other sugars, they just
are not as accurate. Here is a link from fisher scientific which covers this topic
During fermentation, I use a hydrometer due to alcohol impact. Yes you can estimate it with a refractometer, but its just not
as accurate. A correlation exists but its not perfect so refractometer is not as good a measuring device particularly when alcohol
Bottomline is that a refractometer is an indicator of gravity measurement. It is more accurate when alcohol is not present but
gravity measured directly with a hydrometer is more accurate.