There are several reasons why SageMath cannot use Debian's maxima package at the moment
and this second package maxima-sage is needed:

 1. Lisp compiler

 The maxima package in Debian uses only GCL as common lisp compiler, while SageMath uses ECL. SageMath
 cannot use the GCL version, since the preferred interface to Maxima is via an ECL fasl library.
 The possibility of adding an ECL version of Maxima to the maxima package was discussed in

 https://bugs.debian.org/779804

 2. Version mismatch

 Since adapting SageMath to a new Maxima version is a non-trivial process, each version of SageMath
 needs to be accompanied with a specific version of Maxima. Having a specific Maxima package for SageMath
 allows us to provide the Maxima version that SageMath is using.

This maxima-sage package is coinstallable with the maxima packages in Debian.