Why do Muslims go to the mosque?

A mosque is a Muslim house of worship. According to salat, one of the five pillars of Islam, devout Muslims are required to pray five times a day. Although this prayer is permitted to be done from anywhere, many Muslims prefer to do so in a mosque.
At the mosque, prayers are usually led by an imam, or prayer leader. Most mosques have separate areas of worship for men and women. There are no pews or seats in a mosque. Instead, people sit and kneel on carpets arranged to face Mecca.
It is customary to wash oneself before prayer. Outside many mosques there is a fountain where the faithful ritually wash their hands, arms, face, and feet in an act known as wudu. This isn't just good hygiene; it also allows worshippers to enter into prayer in a spiritually clean state as well.
Many mosques house books related to Islamic dogma, law, tradition, and philosophy. These books, in addition to copies of the Koran, are available for study. It is common for mosques to host educational programs in which people read and discuss these texts and their ideas. Many mosques are also home to religious schools known as madrasahs.
Muslims can also go to mosques when they host community functions. Just like other houses of worship, mosques serve as community centers and can hold a number of functions for the congregation.

Prayer is one of the five pillars, or essential practices, of Islam. All devout Muslims say set prayers five times a day (they don't, during these prayer times, pray about whatever is on their minds). In addition to daily prayer, all Muslim men are expected to attend the mosque, the Islamic place of worship, once a week, usually on Friday. Communal Friday prayer is a very important obligation and is called ṣalāt al-jumu‘ah. Devout Muslim women also attend mosque for prayer, though it is not required of them. They pray in separate section or room from the men.
Attending mosque gives faithful Muslims the opportunity to pray with other believers and to learn more about Islam by listening to a sermon by an Imam. The mosque gives Muslims a center of community for their faith lives, just as a church does for Christians.

Muslim people visit a mosque or masjid for the same reason a Christian person visits a church, a Jewish person visits a synagogue, a Buddhist visits a temple, or a Neo-pagan spends time outdoors—it's a house of worship! Most Muslim people visit their local masjid—the mosque in Mecca is considered to be the only true mosque—once a week on the holy day of Friday. Some people go more often, even multiple times a day, for prayer. One of the five Pillars of Islam is to pray five times per day in accordance with the passing of the sun, though people who are ill or menstruating are excused from this. Most people pray at home or work by themselves for their daily prayers and might visit the masjid to participate in the midday prayer on Friday. In the masjid, prayers are lead by an Imam or scholar of Islam.
The masjid also doubles as a community center, where people may gather for a social occasion, further their personal study of Islam, or where children may spend time after school.