Capacity: Capacity Example would be : The amt. of gallons in a pool.
( How much Can Be filled ?sort of questions )
Mass: Mass Example : How much mass does a can of tomatoes have on Earth and the Moon? ( Think the amt. of space taken will be same regardless of weight.)
Length : How tall are you?
( Think how long/ height of it? How much is measures)
Answering your Essential Question : Conversions of measurement can help you solve problems because we can use them to compare, and use them to find the weight or length in a different place ( Think Metric, and Customary Units.)
Hope it Helps! =)