Why people work on weekends? 1.For Extra money ? 2.For promotion ? 3.Because of Company's Policy? 4.Because they love work ?

MOSTLY BECAUSE OF COMPANY'S POLICY.

2. For Promotion i.e Growth and self development

Its all because of the passion to work & to get creative ideas while thinking alone & more room to think aloud. It is one of the ownership quality to think out of the box and stand out from the crowd.

This question doesn't have specific answer, it depends on ones case. 

1) sometimes lack of time to meet the deadlines can be the reason.

2) getting promoted because of extra hours worked is not a fact, unless the employer didn't ask personally(in that case your efforts will be noted for future references), otherwise you shouldn't rely on that, since employer might not always recognise the  efforts which he/she didn't ask to do.

3) For organisation policy: there is a limit on maximum hours of work per week assigned by the Ministry of Labor in each country. The weekly maximum (referring to weekend extra work hours) can be exceeded only if there is a written agreement between the employee and employer. Meaning, in agreement there will be a point where you agree to work extra hours if needed. But that is not a fact, that you will be recognised for your efforts, as it is part of agreement which you signed to follow.

4) Love to Work (i like to call them Workaholics): they will work extra, to show that they are highly energetic and efficient workers  and should be considered for promotion at first hand, but by the time their efficiency ends, as, people physically  Fatigue and Exhausted, which leads to often mistakes and inaccuracy in their reports.... 

5) Another reason is, being employed in two jobs at a time, and working par-time in each. by that, the person can work on one job on week days, and on the other job on weekends.

because there wife is so cruel.and the don't want face her whole day at home 

For me, people work on weekend because of company policy.

some do work on week end because of less payments so they try by all means like working overtimes on week end so that they can fill the gap

It could be All; It depends on employee's circumstances and company policy !