Creating data can be expensive, in terms of the monetary, human, and time resources required. Recreating can be even more expensive and in some circumstances impossible. Your data is precious, not only to you as the researcher, but potentially to others involved in its creation. Indeed in certain circumstances loss of data can be health or even life threatening to survey participants.
Data management planning (and the associated plan document) help to give you insight into the nature of your data set, that is the size, format, sensitivity of the data, as well as allowing you the opportunity to give cognisance to how you will store, secure, and perhaps share the data. Crucially it will also highlight any additional resources (money, expertise, equipment) you might need and a cogent reason for seeking additional funding.
Planning for your research data and its management should start at the earliest possible stage (remember you might be going back to a funder or budget controller), and should continue throughout the research lifecycle. Whilst a data management plan should be one the first deliverables of your project it should not be considered finished until the research project itself is finished. The plan should give you pause for thought throughout the project and may need to be updated to take account for slower (or quicker) progress on data creation, higher (or lower) volumes of data, new formats, or any other unforeseen changes to the nature of the data. It should not be considered a "rule book" that must be adhered to rigidly. If, for example, your data set ends up twice as big as originally envisaged, then the plan can be updated to reflect that, as long as all of the other considerations (e.g. storing, backup, preservation of a bigger dataset) are taken care of.
Creating a data management plan can seem daunting, staring at a blank page at the very start of a research project (having drawn the short straw to be the the team member to "write the DMP") but fortunately the following tabs in this section of the libguide along with the excellent 'DMPonline' tool from the digital curation centre in the UK can help to demystify the process and break it down into much more manageable chunks.