A cheat sheet is  basically an easy reference tool that provides simple, brief and easy to remember or revise instructions for accomplishing a specific task. They often contain formulas, equations, important points, etc. from specific areas. The are termed cheat sheet as these may be used unfairly in exams without the knowledge of the teacher or the professor assigned. 

While there is not one clear way to say if its good or bad practice to allow cheat sheets because there are a lot of things to keep in mind and a lot of people to consider, it generally can be helpful in applied sciences and for subject where the guide does not wish for their students to have to memorise things. 

If  the teacher does not want to gauge the students’ ability to blindly memorise stuff and remember it in a stressful situation because it might not even be relevant to their lives and not what their course is about. The cheat sheet thus replaces the procedure of blindly memorising certain stuff. Moreover, what they write on a cheat sheet is then a good emulation of what the students have available through Internet searches, books, and documents or notes similar in their non-exam situations.
However, the most important part of memorising before a standard exam is structuring and reiterating the relevant and available knowledge as this strengthens the actual understanding of concepts. Making the cheat sheet handwritten replaces this practice. The same applies to mechanically created cheat sheets, as they can simply be a result of copying and pasting. This practice does prevent students from wasting their time on an overly long cheat sheet, it may even increase the perceived fairness of the process. Finally, there will always be some person who blindly copies the solution to every exam task ever given for the subject.