with set analysis you can manipulate the calculation of a formula
eg
sum (sales) gives you the sum of the sales, depending on all selected values
sum ({<year={2014}>}sales) gives you the sum of sales from all other depending selections but only from year 2014
sum ({<year={2014}, region ={'U.S.'}>}sales) gives you the sum of sales from all other depending selections but only from year 2014 and for the region U.S.
set analysis is like adding a condition to an expression, if you are familiar with SQL then you can say that the set analysis represents the 'Where' clause in SQL.
for example, you have a data model which represents country, city and amount.
if you create a chart with dimension country and expression sum(amount) you will see the total amount by country.
but what if you want to select a specific city in the country, then you need set analysis, and you expression will be sum({<City={'CityName'}>} amount)
