The Singleton design pattern solves problems like:
How can be ensured that a class has only one instance?
How can the sole instance of a class be accessed globally?
See Applicability section for all problems Singleton can solve. See Solution section for how Singleton solves the problems.

• The standard way is to call the public constructor of a class (new Class1()) each time a new object is needed.
• That's the kind of approach to avoid if we want to ensure that a class can be instantiated only once (has only one instance).
• For example, system objects that hold global data (like database, file system, printer spooler, or registry). [BGI]
  It must be ensured that such objects are instantiated only once within a system and that their sole instance can be accessed easily from all parts of the system.
• For example, avoiding creating large numbers of unnecessary objects.
  It should be possible to avoid creating unnecessary (duplicate, functionally equivalent) objects over and over again (to avoid excessive memory usage and system performance problems).
  "It is often appropriate to reuse a single object instead of creating a new functionally equivalent object each time it is needed." [JB08, p20]