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If word is not set, then if [ $word != "" ] will resolve to: if [ != "" ]. As a result bash will throw an error ([: !=: unary operator expected) because there is nothing to check against.
If word is set, then there are values to the left and to the right of the check (!= in this case) and bash can do its thing.
The -z option is specially made to check empty (zero) variables.
You can use grep to check if a string is present in a file: grep "something" infile. See grep's manpage for details.