An appropriate problem is one that has these attributes
Given the original contribution requirement of research, one of the expectations is that you will eventually know more about this problem than anyone else, i.e. that you will become an expert. This means that you should have spent a considerable amount of time reading in the problem area and becoming notably familiar with the current literature, including knowledge of strengths and gaps. Reflecting and pondering on the problem area, including some consideration of likely independent variables is important.
Clearly, research requires considerable time and effort, especially if you hope to find reasonably large generalizations. You need to know exactly which steps are involved in the research process and what they require. Knowledge of the appropriate research methodology is particularly important.
A proposal is similar to an outline and surely you have learned how useful it is to create an outline before you write a paper. The proposal is a detailed outline for your research project. As a draft, it is relatively easy to ask for suggestions for improvement, see if journal editors might be interested in the resulting publication and the like. Since the proposal/outline is not yet final, there is ample opportunity to revise and improve.
A well-done proposal will insure that the research is conducted efficiently and economically. A well organized and well written proposal is easily read and makes gathering suggestions for improvement relatively easy. Snags, problems, and missing elements are soon identified.
Another way to look at the proposal is to see it as a pilot study or trial run for the larger research project that you hope to do later. A proposal that includes some test data collection is likely to discover problems and opportunities while there is still time to revise and change.
Since preparing the proposal involves some time and effort, the proposal indicates that there is a serious commitment to the research enterprise and not just a casual interest. This is particularly true in academe, where a proposal must be approved before you can register for thesis or dissertation hours. The better the proposal, the more likely that the researcher will find faculty eager to participate in the committee. Funders normally require a substantial research proposal before they will consider an application for funding. Book editors will require a substantial proposal before considering a research-based manuscript.
The research proposal is a mini version of the research article, thesis, or dissertation. Thus, it contains each of the elements found in the parent document.
When the proposal is approved or accepted, you begin your research project with more confidence, especially if a committee of knowledgeable faculty have indicated their enthusiasm for your work. Since approval represents a sort of contract, you need to consider how to handle changes since some change is likely as the research unfolds. Note that substantial changes will mean a return to the approver for another approval.