An oechsle meter or hygrometer as it is also called is used to see how much sugar there is in the wine when brewing. It is useful to see how strong your wine will be before you have started the fermentation. By reading the value of the hygrometer and compare it with a table of alcoholic strength you can determine if you have enough sugar in the grape juice for the strength you want to achieve. It can also be used to see if the fermentation is complete.