A software product is developed for many reasons. Again a software product maintenance is done for various reasons. For doing any software development or maintenance, a project is created. Project team develops or maintains the software product. In doing so they may follow some processes. Some of these processes could be termed as project processes. For example, the project manager may create a project plan and assign tasks to the project team members. the project team members could do things like making requirement specifications, make models of the software product or do coding or do testing of the constructed software product.
In doing any of these activities, they may follow their way of doing things. or the product owners or the project manager may have defined some rules which are to be followed for doing any of these activities.
Suppose, each project team member is following his/her own method of doing things. for example, a software engineer likes her own kind of writing & compiling code and does not bother about how other software engineers are compiling their own code. This will result in code written in different styles. Even if her code compiles without errors, still this will result in creation of an unstable system. this system will be difficult to maintain. The other issue here is the use of compiled libraries. Many compiled libraries are developed by project teams and they need to be used in writing further code. If team members are not using already created libraries then creating source code again & again for the development of same features results in unproductive work ethics & waste of time.
When a software product is being built or maintained and if it is small work and only one person or a few people are doing it then following some standard method of doing things is not required. However most projects for software development or software maintenance are large efforts and require many people to it. In this case, some well established methods of working must be followed. Each project team member must stick to these methods.
These methods together can be termed as software engineering methods.