Why was the register keyword created?

register

In C, the register storage class was used as a hint to the compiler, to express that a variable should be preferentially stored in a register. Note that the hint to store a register variable in an actual register may or may not be honored, but in either case the relevant restrictions still apply. See C11, 6.7.1p6 (emphasis mine):

A declaration of an identifier for an object with storage-class specifier register suggests that access to the object be as fast as possible. The extent to which such suggestions are effective is implementation-defined.[footnote 121]

[footnote 121] The implementation may treat any register declaration simply as an auto declaration. However, whether or not addressable storage is actually used, the address of any part of an object declared with storage-class specifier register cannot be computed, either explicitly (by use of the unary & operator as discussed in 6.5.3.2) or implicitly (by converting an array name to a pointer as discussed in 6.3.2.1). Thus, the only operators that can be applied to an array declared with storage-class specifier register are sizeof and _Alignof.

In C++ it is simply an unused reserved keyword, but it’s reasonable to assume that it was kept for syntactical compatibility with C code.

auto

In C, the auto storage class defines a variable of automatic storage, but it’s not usually used since function-local variables are auto by default.

Similarly, it’s reasonable to assume that it was initially carried over to C++ for syntactical compatibility only, although later it got its own meaning (type inference).