When you buy an apartment - or sometimes a freehold house - within a development, there are common items that need to be maintained. In an apartment block, the hallways need to be cleaned, a lift will need to be serviced and the exterior of the building will need to be repainted from time to time. In a housing development, gardens and possibly security gates will need to be maintained.
All of this will usually be documented in a lease (for an apartment) or TP1 (for a freehold house), and all of these obligations need to be organised by someone. A maintenance plan needs to be developed, a budget needs to be prepared, funds need to be raised and various tradespeople need to be contracted, all managed in accordance with the lease and the law. This is the role of the estate manager or managing agent.