At the federal level, the FEC requires public disclosure of employer information for donations over $200. At the local/state level, many jurisdictions require this disclosure for donations at this amount or even smaller.
We follow the industry standard of asking all donors for this information in case people make multiple donations that hit this threshold.
The FEC discusses this in more detail here:
If you contribute more than $200 to a committee, the committee is required to use its best efforts to collect and publicly disclose on a financial report your name, address, occupation and employer, as well as the date and amount of your contribution. Committees sometimes request this information even for smaller contributions, since the $200 reporting threshold applies to your total contributions to one committee during a calendar year. For example, you may make several small contributions to a committee during a year. Once these contributions add up to over $200, the committee must report the contributor information.
Note that if you collect and forward contributions to a committee, you must transmit them within a specified period of time and must also provide the committee with certain information on the contributors. Additionally, you may have reporting obligations.