It is important for you to first understand what Shift-left actually is:

Shift-left is basically a practice implemented to find and prevent major defects earlier in the software development process. DevOps recommends Shift-left testing practice so as to bring the testing phase in the earlier stages that saves a lot of time in the development process.

The traditional method gets the testing phase around the end of Software Development Life Cycle. This increases the time required to re create software instances if a major testing error occurs. With the help of Shift-left practice, these errors can be rectified much earlier in the SDLC and will save time, efforts, and money for the organization.