What is the use lease deed for 999 years?

I am from pune, Maharashtra, I am in process of purchasing a flat which is in a co op housing society. Housing society consist of small plots around 150 where standalone apartment buildings are built by different individual, apartment in which I am buying a flat is one of them. Builder has provided me some documents for banking purpose, where it was mentioned as- 1] land is given to 'X' person on lease of 999 years by housing society then that 'X' person given same land to 'Y' person for same lease and then that 'Y' person got that land developed from builder. 2] Builder is selling those flats with consent of this 'Y' person, where it is mentioned that land is for lease 999 years 3] my questions are as- i] what is the use of this lease of 999 years, why not just sell the land ? ii] If I am buying flat from builder then what will be my title? am I owner or something else? iii] if I am buying flat then what are particulars I should be looking or ask for same to builders? iv] if I am going ahead with purchase process then will it be safe purchase or there will be any issue while resale or re-development? I am not involved any legal activities earlier, so forgive about language or explanation. Thanks in advance.