Question

we can travel by land .we can travel by air(do combine)​

Answer 1

Answer: We can travel by air or by land.

Answer 2

We can travel by land. We can travel by air. (combine sentences)

We can travel not only by land but also by air.

When we combine sentence with ‘not only--- but also’, it is called a correlative conjunction. In such sentences both sentences have some similarity as well as difference. To highlight that difference, we use ‘not only--- but also’.  Both the sentences have equally important information.

Here, emphasis is given on the last parts of both the sentences – by land and by air.  ‘Not only--- but also’ is used  to highlight the emphasis.