In general, a broker (who is frequently part of a larger firm known as the brokerage) is required to allow potential buyers access to view the property. While it is not a strict requirement for a buyer to view properties, they may not be able to get the same levels of access without them.
A buyer may choose to view properties online, which does not necessarily require the use of a broker. Some of the properties may be difficult to find online, and you will not have as much detailed information as a broker.
Alternatively, you could speak with a seller directly to obtain additional information or to gain access to the building itself. While this is an option, not all sellers will be available at times that are convenient for you, and some will only allow buyers to visit through their broker.