Employees are useful for people who have large herds. The employees will give you checkbox buttons in Herd Management under the Care tab that you can check once to select all your horses that need food/water/groom and then click the "purchase" button. This eliminates the needs to click one by one or visit the horse page and go through them all. 

I definitely say early on in the game the food/water employees are more beneficial because you only need to groom when you plan to breed your horses as it only affects breeding outcome. 

Tips on the construction area would just be to save up your money and only buy the 25 stall blocks instead of the smaller ones. Then you don't have to worry about demolishing the small one laters and only recovery some of your money. 

Another thing in construction area would be the food/water storages. If you plan to keep your herd smaller, they can be useful to auto feed/water your horses. Then you wouldn't need the feed/water employees. I found with my large herd the amount of acres the storages used wasn't worth it. I would rather have those acres used for stall blocks so I can board more horses at home. So it really depends on your playstyle how big you want to go with your horses. 

Thank you for the complete info! I think i'm gonna use them because i already have over than 100 horses and i don't want to sell them right away. Definelly a big herd.