I have been in engineering management and planning capacities several times in my career.
The purpose of planning is to match what you want to achieve with the resources it's going to take to get it done. I've seen many examples where failure to adequately consider resources caused projects to fail. Which results in a lot of discouragement for both the team and the company.
In reality, planning comes in three phases:
The long-term strategic plan or technology plan. This is usually a collaboration between Product Management, R&D, and engineering leadership. The desire is to have a "roadmap" of what technologies are possible with some sort of "desirability" score or "applicability" score for the technology. This often has some sort of R&D stage-gate evaluation process that seeks to answer key feasibility questions before increasing the amount of investment in that area. When technologies get through this process, they should be ready for commercialization through a project.
The project plan. This is the project that takes technology to market through design and commercialization. This will usually be kicked off by Product Management or Product Planning and handed off to Project Management. It's the plan to take feasible technologies from the R&D org to a commercially producible product. This will often span several years and have a cross-functional stage-gate process that ensures all of the operations, finance, marketing, engineering, service, sales, etc. functions are working in sync to launch the product.
The Annual Operating Plan. This is the next year's overall plan and budget for the business. Often it's led by finance, but it's always agreed to by senior leadership. This is the plan of how much money to spend, and resources to apply to each project in the project management list. Often there are significant prioritization trade-offs that have to happen here. There will be top priority projects that are considered the most important for the business, and there will be projects that are partially funded because they're not as important. In a mature organization, partially funded projects will have relaxed expectations on timing, scope, or quality.