How often do we see a perfectly good idea fall flat through the responsible person (if one indeed exists) not having taken care in the planning process? Whilst it is not feasible to foresee all possible obstacles to success the most potentially damaging can be avoided or mitigated by careful planning.
Any initiative that is perceived as providing value to the business and therefore worth doing is also worth the effort to plan carefully. Doing so allows for a better quantitative understanding of the benefits it may bring, the impact it may have on other related initiatives and the resources it may consume in its development and execution.
Not planning can result in unexpected costs, resources conflicts and business impacts that are undesirable. Scenarios arising from not planning range from little impact (because of good luck), failure of the initiative and with the the worst case, failure of the business itself.
Having established planning processes and following them is absolutely essential if outcomes are to be optimised. Anything less is just a gamble.