Whenever a civil lawsuit commences in state or federal court, the parties to the lawsuit gain subpoena power. The power is found in Rule 45 of the Federal Rules of Civil Procedure. This means that they can use court authority to direct witnesses to testify or produce documents that may be, or may lead to, admissible evidence.
This power does not apply to other parties. They are obligated under different rules to make facts and documents available. (Rules 26, 33, 34 and 36 for example).
A subpoena also acts as a tool allowing a party to obtain confirmation that the opposing party is producing all that is requested of it. For instance, in a prevailing wage case by a former employee of a subcontracting construction company, a subpoena to the general contractor for proof of all prevailing wage contracts with the sub in the relevant time period forces the construction company to diligently produce accurate records.
This is so because the former employer does not know what the contracting company will produce. Unless they are colluding. Not very likely that. Companies and people not party to a lawsuit like to keep it that way.
Subpoenas issued to vendors and customers of a corporate defendant are unnerving to all concerned. Responding or objecting to them can be expensive. The effort to object or the ease of compliance can be telling too. In addition to their discovery value, they increase pressure on parties to more strongly consider the value of settling earlier.
Use subpoenas early to make sure you are getting a complete picture of the other party and its claims or defenses. If other benefits flow from the effort, then so be it. There is of course the risk that your subpoenas do nothing but irritate otherwise helpful witnesses. This is a real risk, so consider the costs you may incur before flexing your discovery muscles.
of the completeness of production required of opposing parties.
