How would having access to land trade routes help Islam to spread quickly?

In addition, according to historical accounts, Muslim traders came to the Indonesian islands because of the rare spices present there. It is believed that some of these merchants settled in Indonesia and blended with local people. Moreover, after the arrival of Muslim merchants in Sumatra Island, the kings of the island started to follow Islam, which further facilitated their integration into the trade roads around the 12th century AD. Archaeological evidence of the adoption of Islam amongst the royalty can be seen on tombstones engraved with the date of the Islamic year of Sumatran Kings of the 13th century.

a.) are both appointed for life