This cannot be stressed enough.
Stakeholder buy-in is required to ensure that a project's planned change is implemented, it is therefore vital to its success.
As a responsible project manager (PM) obtaining buy-in is also the right thing to do as a project will typically affect several stakeholders, i.e. those who have a vested interest in the project. Positive engagement will almost always result in better outcomes of a project as a result of shared knowledge and feedback.
Stakeholders can be directly or indirectly affected by the project, making them primary or secondary stakeholders. They are also categorised as internal and external i.e. individuals within the PM's organisation and those outside of the organisation.
Lack of buy-in from the following stakeholders can affect a project in the following ways:
Senior management can stop the project from getting off the ground by not providing necessary approvals;
The project team can cause delays to the programme through slow action, which will result in cost implications;
Middle managers can prevent enthusiasm for the change among those below them;
Employees on the front line can delay or prevent implementation and create long-term obstacles to the initial and/or continued success of a project;
Other interested parties such as pressure groups if 'successful' can also cause a project to change its outputs or cause it to be shelved.
Intended end users can deem the project a failure if the product or service is not fit for purpose or providing a benefit. Arguably, a project's success is dependant on how it is valued by the end users.
The absence of enough stakeholder buy-in, regardless of the level of investment, can therefore relegate a project to a lessons learnt report.
To summarise, the more support for a project from the various parties, the easier the PM's role and the more rewarding the project!
There are many facets to this topic but some further information can be found here - How to Get Stakeholder Buy-In | Lucidchart Blog