By getting an greencard you will acquire green signal for Permanent visa in USA.It makes you in to position of an permanent resident in usa as well as you get full legal rights to work in USA.And thus getting an greencard it makes you eligible for applying for citizen of usa. And also greencard does not interfere your present citizenship.
When you get the greencard it makes you go anywhere and comeback to USA for your any requirements such as from your job to higher studies to setting up an business in USA to all means as you will be treated and given rights as equal as an american citizen it does not cancels or crashes.
Out your Previous country citizenship and hence you can startup your own business or expand yours without any problems or interference with ease.
And thus greencard provides you benefits as an american citizen without cancelling your native countries citizenship.