Usually because the frequencies at one or both ends of the distribution are very small.
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The most important thing in creating intervals for a frequency distribution is that the intervals used must be non-overlapping and contain all of the possible observations. They are often equal intervals, but sometimes unequal ones are used. It all depends on the data.
To make the graph symmetrical
they may be necessary in certain situations to avoid a large number of empty ,or almost empty, classes.
Rhombus
A scalene triangle.