Reason for purchasing it is its low price(here reason is used as uncountable) There are several reasons for it (here reason is used as countable)

The reason for buying it is its low price.( Uncountable) There are several reasons for it(countable)

The Oxford Learner's Dictionaries website is an excellent resource. https://www.oxfordlearnersdictionaries.com/definition/english reason noun https://www.oxfordlearnersdictionaries.com/definition/english/reason_1?q=reason meaning 1 [countable] a cause or an explanation for something that has happened or that somebody has done reason (why…) I'd like to know the reason why you're so late. Give me one good reason why I should help you. meanings 2 and 3 skipped meaning 4 [uncountable] what is possible, practical or right I can't get her to listen to reason.

Countable - the explanation for something e.g. the reason I bought it was; one of the reasons is that... Uncountable - the ability to think e.g. reason says that his hands are dirty because he has been playing in the mud.