The fact that no devices have yet been found also suggests that the purpose of the threats was to create disruption rather than casualties.
Terrorism in Japan is a phenomenon that has a long and storied history, but one element that has been constant in each incident is that the perpetrators have all been Japanese citizens. From the exploits of the prewar Genyōsha, to the activities of the Japanese Red Army in the 70s and 80s, right through to the Sarin gas attack on the Kasumigaseki station in 1995 by Aum Shinrikyō, all of these organizations have been formed and attacks carried out by Japanese nationals, although the targets have not always been Japanese citizens or institutions. In this instance, since the threats have been limited to specific targets, and given that warnings have been issued beforehand, suggests that they have been done for nuisance value, distracting law enforcement agencies from other tasks and irritating commuters and tourists but not really intended to cause destruction.
Until the perpetrators are caught and their motives revealed, we can only speculate as to why these threats have been issued. They may be related to President Trump’s visit, they may be related to the recent election victory by the LDP/Komeito coalition, they may be an act of a left or right wing activist group intent on sending a message to authorities, they may be the work of a disgruntled citizen or group of citizens who object to some development in their local community, or they might be a case of simple harassment. It is unlikely to be terrorism inspired by religious teachings, and certainly not extremist terrorism, although the targets do have common characteristics with incident sites in other states. The use of a bomb threat alone distinguishes these threats from those made by extremists in other countries, although we do not yet know the details of the threats. What we do know is that they were made in Japanese, directly to Japanese institutions, and so point towards the standard pattern for terrorist threats in Japan.