A common tactic used during the cold war was known as Probing. You would send in an asset of some form (a long range bomber, for example) and then watch how your opponent responded.
This might mean observing how close you could get to their territory before being detected, and how long it would take for their interceptors to respond.
After the Gulf war it was common for the US to fly close to the Iranian border in order to goad the Iranians into targeting them with their air defense radars, as a method of determining where they were located.
It has been hypothesised that the devices in question may have been launched purely as a way of determining how long it would take for them to be spotted or what the response to them might have been.
In his book Open Skies, Closed Minds, British author and former defense analyst Nicholas Pope states that NATO radar systems often have filters on them to cut out noise, so that it is easier for the radar operators to see potential hostile aircraft.
For example, they would often automatically exclude fast moving objects at extreme altitudes as meteorites entering the atmosphere, and slow moving low objects as ground clutter caused by trees or birds. He posited that this may be creating blind spots in which hostile aircraft or spying devices could be hidden.
It may be the case that the device was being used to determine what the tolerances of the US filters where, and whether they automatically cut out devices of this nature.
We know that this is a possibility as news reports from the time stated that once the military went back over old records they found several more similar devices, and they then found several more current ones. Meaning that they may have been filtered out.
There has also been speculation that these devices may not have been intended for espionage in the traditional sense, and that they may have been used as mapping tools in order to create maps of the sea bed or of ice flows (Some passed over such areas before being detected). During the early 2000s it was discovered that Chinese submarines were able to operate in areas close to Japan without being spotted because they had extremely accurate maps of the ocean floor. It is suspected that fishing boats were used as a cover for mapping missions.