I test the sap so that I can better plan my sugaring, for example:

* If I get a consistently low tree, I will not bother with it.
* I will most likely toss sap less than 1%.
* If I get a superstar tree, I'll put in two taps if there is enough diameter to warrant it.
* If the sap in my storage pails is really low in sugar content I'll freeze it for several days in a row and chuck the ice from the top. Last year I did that with 15L of low sugar sap. After 3 days of 'chucking and checking', what I had left was 8L at 5.1%.
* Knowing how much sugar I've got helps me to know how much propane I'm going to run through so I can plan on a good time to run for refills.
* Knowing the sugar content lets me know in advance how many hours I need to plan for my boiling sessions.

etc etc you get the idea.

But, most importantly, it's really fun to be checking your trees' sugar every day. Honestly, it really is.