What is the significance of juxtaglomerular apparatus (JGA) in kidney function?

The juxta glomerulus apparatus is plays a major role in the regulation of the rate of the glomerular filtrate. It is a special sensitive region formed by the cellular modifications in the distal covuluted tubule and afferent arteriole at the location of their contact. 
The JGA plays a complex reulatory role. It regulates the complex mechanism called the renin-angiostensin mechanism. The JG cells are activated when there is a fall in the glomerular blood flow or GFR. On actibations these cells release renin which converts the inactive angiotensin to angiotensin- I and further to angiotensin-II. 
Angiotensin-II is a powerful vasocontrictor and thus increase the glomerular blood pressure thereby the GFR. It also activates the adrenal cortex to release the hormone aldosterone. 
Aldosterone directs the reabsorption the Na⁺ and water from the distal parts of the tubule. The reabsorption leads to increase in the blood pressure anf the GFR.