Expungement of your records will allow you to "eliminate" traces of any criminal conviction in your past. Having even a DUI conviction on your record, can possibly interfere with your prospects for future employment, or further studies. Getting your record expunged, in many states, allows persons to get convictions for minor crimes erased from the record, in order to avoid any effect of these convictions on their future.
In most cases, you do not have to reveal your expungement. However, these rules to vary from place to place. Broadly however, you do not have to reveal your expungement when you apply for a job, or when you are in line for an apartment rental.
However, in some states, you may be required to reveal your expungement, depending on some situations. For instance, some states will require you to reveal any possible criminal record in your past, even if these have been expunged, if you are applying for a law enforcement job. These don't have to be federal law enforcement jobs. Even if you're applying for a local police officer position, you may have to reveal the expungement. You may also have to reveal your expungement if you are planning to apply for a state-issued professional license.
In some states, candidates who are applying for school-related positions, both public or private, are required to make their past criminal record completely clear. Additionally, if you're applying for a firearm license, or a concealed-carry permit, you may be required to make your expungement clear. Also, if you plan to apply for public office, you will be required to disclose all expungements.
Disclosing expungement, however, does not automatically mean that you will be barred from running for office, or buying a firearm.