Why the reverse charge exists
When a non-resident supplier sells services to a UAE business, the supplier is not registered here and does not charge UAE VAT. Article 48 of the VAT Decree-Law places the obligation on the UAE recipient instead: the registered business calculates the VAT, reports it as output tax and, if the service is used for taxable supplies, recovers it as input tax in the same return.
For a fully taxable business the two entries cancel out. The point of the mechanism is that a UAE business buying from abroad is treated the same as one buying from a UAE supplier, and that businesses with exempt activities cannot avoid VAT by importing services.