It helps involve product stakeholders early when product roadmap is being created. In some organisations, product manager need to align all product stakeholders before finalising product roadmap.
This gives product managers an opportunity to ask for product stakeholders’ feedback and address their feedback.
Also, as and when updates are made to product roadmap , it is good to involve product stakeholders and share updated product stakeholders to product stakeholders.
The initial and ongoing alignment to product roadmap with engineering and other teams are very important. At the end if product roadmap item milestone is not met , you do not want engineering leadership to say that he/she knew on day one that product roadmap items milestone cannot be met by engineering team.
Also this helps to get development time tentative estimates. These estimates are not accurate but lay a good foundation on timelines and better planning
Similarily at the end , you do not want Sales head to say that they did not sales target as features built was not the features customers wanted.
Hence you as product manager should share initial product roadmap and get alignment and buy in from all product stakeholders.