Sales Module gives you access to your Customers: contacts, addresses, work done …
You can get infos about prices and change them
Add new sites
Add / change prices
Quality Control: know about problems.
CRM: support your customers, visit them regulary
If your customer calls you and asked when the Driver will be there you can get the last sent location on Map and how far away he/she is from your customer.
If you want to schedule a route to some of your customers, you can see locations on the map which helps you to organize it.