Salesforce Lightning Design System (SLDS) gives you the tools to construct apps that follow Lightning Experience's CONCEPTS, design language, and best practices. 

The following are some of the advantages that make SLDS so beneficial:

• When enhancing existing functionality or integrating with external systems, gives a unified experience and streamlined workflows.
• Padding and MARGINS are not over-enforced by default.
• It is updated on a regular basis. As long as you're using the most recent version of SLDS, your PAGES will be Lightning Experience compliant.
• The CSS framework includes accessibility.
• It is compatible with various CSS frameworks, such as Bootstrap.
• Visualforce pages that match the look and feel of the Salesforce mobile app can be created with the Lightning Design System (SLDS). To use SLDS, you'll need to make a few changes to your code and remember a few things. Visualforce programming that uses SLDS, for the most part, works without a hitch.
• Our action, custom, DOCTYPE, standard, and utility icons are available in PNG and SVG (individual and sprite map) FORMATS in the Lightning Design System (SLDS).