The Suleymaniye was built as far more than a mosque, and its architect chose to be buried close enough to it that he is, in effect, still part of the complex.
A charitable complex, not just a prayer hall
The Suleymaniye was constructed as a kulliye, an integrated Ottoman charitable complex combining the mosque itself with a hospital, soup kitchen for the poor, schools, a caravanserai for travellers and a public bath, reflecting an Ottoman model in which a major mosque functioned as the anchor of an entire social welfare system rather than only a place of worship.
Two tombs, side by side in status
Within the complex's grounds, a mausoleum holds Suleiman the Magnificent, the sultan who commissioned the project, and a nearby, deliberately more modest tomb holds the architect Mimar Sinan himself, who is credited with designing his own tomb as part of the same complex he built for his patron, placing the builder's resting place permanently alongside his most celebrated work.
Answering, then surpassing
Sinan is widely understood to have treated the Suleymaniye's design partly as a direct architectural response to Hagia Sophia's dome, refining structural solutions for the dome's supporting piers and buttressing in ways later Ottoman architects and Sinan himself built on further in subsequent commissions across the empire.