The main point to a sample is to determine your gauge. Every knitter knits slightly differently. One knitter gets the recommended gauge using the yarn and needle specified in the pattern, the next one does not.
On some items gauge isn’t really important, but when you are knitting garments like sweaters and socks it is critical. After all the work and time learning you made a sweater that fits your 6 yr old niece but was intended for yourself is very frustrating.
You don’t have to bind off the sample or complete it. Knit enough (usually about a 4 inch square in garter or a common pattern stitch based on your pattern) and then measure it, count your stitches and your rows and compare it to the pattern’s gauge.
I hope that helps.