Yes both Income and Domicile certificate are important for engineering admissions in Maharashtra because of some conditions:-

1.To get admission under the Maharashtra State Quota a Domicile Certificate is mandatory. It proves that the candidate is a resident of Maharashtra.

2.If you are applying under the scholarship schemes, then an Income Certificate is compulsory.

3.without a Domicile Certificate  you are treated as an all India Candidate, and you cannot claim reserved seats or benefits meant for Maharashtra students.

4.if you are not applying for TFWS or EWS, then the Income Certificate is not compulsory

5.During the CAP rounds these documents are checked strictly.

Hope it helps.