this tool is extremely valuable in trending long term performance data. i use it with the same test file sizes every time. i also try to test at close to the same time of day each time, but also try to take data at different times of day too to sort of randomize the data. When you look at the average over 1, 7, 30, 90, 180, etc days and compare the averages you can see if you're trending down, flat or up over time. and since you can select on the plot points, you can look at the problem points to see what time of day the problems were occurring. When the trend is down, it's potentially an equipment degradation or it's showing you an issue with the ISP; either an overload of customers on your local route or on one of the ISP's rented lines from other companies. The time of day data is helpful in clearing your equipment i.e. when you have certain times of day when the speeds are very good and stable, and another time of day it isn't, then it's not likely a problem with your equipment (although that's not 100% certain as big temperature swings can aggravate hardware problems if they're there).
there's certainly lots of other value in the tools here other than trending the speed database, especially when there is a problem and you're troubleshooting. maybe it was already obvious about the trending, if it was, this is just a good plug for CA3LE's site.