The two primary values from INCOSE Certification are those gained by going through the certification process and those gained by having (and advertising) the certification. 

Going through the certification causes you to reflect on your systems engineering work experience and to expand your SE knowledge. These steps are useful even if you don't get certified, but it is hard to motivate yourself to read the INCOSE SE Handbook if you aren't being tested on it. 

Telling people you are certified, or listing it on your resume, helps people understand what your skills are interests are. You're a person who has chosen to identify as a systems engineer and has external validation that they're capable at it.