What is the purpose of aggregation in view?
Without aggregation, Views fetches data from the database and lists that data in individual "rows". 

When you turn on Aggregation some mathematical functions can be applied to the existing results under certain conditions. That is how you might get the SUM of all the values of a field, or the AVERAGE etc. 

Charts usually don't display individual "rows" of data, they display statistical data that you get from calculating some values from the entire results list. That is why charts need Aggregation - to get to the numbers that are shown on the chart.