Why did abbasid's have mercenaries?

The Abbasids employed mercenaries primarily for military and political reasons. As their empire expanded, they faced challenges in maintaining loyalty among their diverse populations and local military forces, which often had competing allegiances. Mercenaries, often recruited from various ethnic backgrounds, provided a more reliable and loyal fighting force, allowing the Abbasids to maintain control and suppress revolts. Additionally, using mercenaries helped to reduce the power of local tribal leaders and military commanders, consolidating the Abbasid authority.