If you look in the file being included, you will see that it contains a number of defines. These defines can make your code more readable. It is never necessary, but using it will help you write more readable and maintainable code.

The XResultSet example looks like this:
vRSHandle = @XResultSetSearch("Customers.db", "Customers", SEARCH_MODE_AND, SEARCH_SYNTAX_QA, "")

In the line above, SEARCH_MODE_AND and SEARCH_SYNTAX_QA are defines provided by sbasic_include.sbas. If you remove the include, this line will not compile. The relevant defines in sbasic_include.sbas look like this:
#define      SEARCH_MODE_AND            0
#define      SEARCH_SYNTAX_QA            2

You can write the same line of code without needing the include file as follows:
vRSHandle = @XResultSetSearch("Customers.db", "Customers", 0, 2, "")

This will compile without the include but, as you can see, it's less clear.