The Kafka client has the ability to start consuming at an offset before or after a given point in time. The granularity of this offset is the log file as the Kafka servers do not keep track of arrival time of various messages.
This means that the granularity of offsets relative to time depends on arrival rate of messages and thus of log file rotation. A topic with lots of messages will have its log files rotated very often (thus each spans a short time interval) whereas a topic with very few messages might not see its log files rotated for hours.
In order to circumvent this granularity disparity, having a parameter that would force log file rotation after a certain delay (xxx ms) would allow for pretty much constant time granularity to be available at the cost of more file descriptor being used.