People have groups of tasks, timelines, commentary on tasks, the need for other people to do things so they can do their tasks, and many, many interactions between tasks and the results of those tasks in SaaS development. You need to have an effective system to manage all those tasks and the documentation that comes along with them. Your Project Manager will have their suggestions on what to use and how to use it.
Personally I recommend using JIRA by Atlassian for development projects, but there are a variety of different systems available.
Other systems I’ve seen used effectively for SaaS development are:
Your SaaS system is going to end up having literally thousands of tickets over time and tens of thousands of comments. If you don’t have a Project Management tool for your SaaS system development, you will quickly start losing things and waste a huge amount of time or the project will fall apart. So spend the money & get the right tools to do the job. It will be a huge cost savings in the long run.