How i can use byte_math only with variable

Rules
1

I want extract 1 byte with byte_extract. Then variable from byte_extract use in byte_math for calculate.
But in documentation says that:
byte_math:bytes | , offset , oper , rvalue , \
result <result_var> [, relative] [, endian ] [, string ] \
[, dce] [, bitmask ];

if i take variable-name from byte_extract, that i don’t need use offset or need to use offset in byte_math?
If i need to use offset anyway in byte_math, say me why? and what offset do? because i take and calculate variable, NOT bytes.