Membership fees and contributions are used for the maintenance and upkeep of the Masjid (mosque). The Masjid belongs to Allah, who charges all worshippers with its upkeep and maintenance.

Allah says regarding the maintenance of the Masaajid,

إِنَّمَا يَعْمُرُ مَسَاجِدَ اللَّهِ مَنْ آمَنَ بِاللَّهِ وَالْيَوْمِ الْآخِرِ وَأَقَامَ الصَّلَاةَ وَآتَى الزَّكَاةَ وَلَمْ يَخْشَ إِلَّا اللَّهَ فَعَسَىٰ أُولَٰئِكَ أَن يَكُونُوا مِنَ الْمُهْتَدِينَ

The mosques of Allah are only to be maintained by those who believe in Allah and the Last Day and establish prayer and give zakah and do not fear except Allah, for it is expected that those will be of the rightly guided.