So you read a book, but didn't understood the reason for the advice in the book?

The "traditional" 8051 chips can sink current but only have a weak pull-up. So when you need to draw the signals high, you need an external transistor or an external buffer chip.

And in some situations, you have a big need to invert the logic of the I/O pin, since you want the output to be low when the processor is in the reset state.

But adding a 74HCxx between the processor and another 74HCxx chip isn't meaningful unless you need to invert the signal, or in case you need to send the signal a long distance and then needs stronger pull-up capability or maybe an input with schmitt-trigger.

It's important when reading books to not pick up suggestions as magic facts, but instead to learn the reason behind the suggestions. Design rules are only meaningful when you know when the rule is applicable, and when it isn't.