Memcheck is the default Valgrind tool when no other tool is specified in the parameters to its executable. Memcheck itself is a memory error detector capable of detecting the following types of issues:

• Accessing memory outside of allocated bounds, overflowing of the stack, and accessing previously freed memory blocks
• The use of undefined values, which are variables which have not been initialized
• Improper freeing of heap memory including repeatedly freeing blocks
• Mismatched use of C- and C++-style memory allocations as well as array allocators and deallocators (new[] and delete[])
• Overlapping source and destination pointers in functions such as memcpy
• The passing of an invalid (for example, negative) value as the size parameter to malloc or similar functions
• Memory leaks; that is, heap blocks without any valid reference to them

Using a debugger or a simple task manager, it's practically impossible to detect issues such as the ones given in the preceding list. The value of Memcheck lies in being able to detect and fix issues early in development, which otherwise can lead to corrupted data and mysterious crashes.