It's akin to a compiler unrolling a loop. Uses more RAM (area) but fewer cycles to execute. Hardware synthesis uses many of the same techniques as compilers use to optimize code.

It's a common pitfall for those learning hardware description languages like Verilog, when they think about them like programming languages. If you go "if (calc) res <= a * b;" If res is 32 bits wide then you have instantiated a 32 bit fast multiplier circuit dedicated just to that one operation. This is often not what was intended.

Despite how leaning on the analogy too closely can mislead in that way, the analogy between hardware and software is not a shallow one. A combinatorial circuit is akin to the pure function of functional programming. Anything that can be described as a pure function working on fixed integers or floating point or other discrete data types, can be transformed into a combinatorial circuit. And there are algorithms to do so automatically and often with reasonable efficiency.

Free software synthesis has come a long way in recent years, by the way. There's even several hobbyist projects that can take VHDL or Verilog and produce layouts using TTL chips or even discrete transistor logic with automatic circuit board layout. You can now compile your code directly to circuit board copper masks and a part list.