Active galactic nuclei (AGN) are the most luminous persistent sources in the universe. A minority of AGN are characterised by powerful plasma jets extending from close to the supermassive black hole at the centre of their host galaxy up to megaparsec scales in the intergalactic space. Inside these jets, charged particles are accelerated to relativistic speeds and emit non-thermal radiation. In blazars one jet is oriented at a small angle with respect to the line of sight, and this causes Doppler beaming of the jet emission, with consequent flux enhancement, and decrease of the variability time scales. The dominant contribution of the jet radiation to the blazar emission makes these objects ideal sources to investigate what happens in the inner regions of AGN jets and even what is the jet structure and dynamics. We have been studying blazar variability for more than 20 years in the framework of the Whole Blazar Telescope (WEBT) Collaboration, including many tens of astronomers observing mainly in the optical, but also in the radio and near-infrared bands. The analysis of the wealth of multiwavelength data gathered during the WEBT monitoring campaigns, with unique time resolution in the optical band, allowed us to outline a model for the blazar variability, involving an inhomogeneous twisting jet. In the final part of the seminar we will mention the contribution to blazar understanding that is expected from Vera C. Rubin Observatory’s Legacy Survey of Space and Time.