Use this to have services remember the current topic and reset it if the room empties and then is reformed. It does not provide any security with regards to whom may set the topic, see mode +t or topiclock to restrict who may set the topic. Keeptopic can only be turned on if topiclock is first turned off since topiclock includes the functionality of keeptopic and also provides topic security.
Example:
/msg chanserv set #services keeptopic on
This is a good command to use if you do not intend to use topiclock. While a topic may not be essential for your room, there is rarely a reason to not keep one in memory. It is a nice aesthetic touch, and if you set useful topics, it can be helpful.