The aim of MOSE is to protect the lagoon, its towns, villages, and inhabitants along with its iconic historic, artistic, and environmental heritage from floods, including extreme events.
Although the tide in the lagoon basin is lower than in other areas of the world (where it may reach as high as ), the phenomenon may become significant when associated with atmospheric and meteorological factors such as low pressure and the bora, a north-easterly wind coming from Trieste, or the Sirocco, a hot south-easterly wind. Those conditions push waves into the gulf of Venice. High water is also worsened by rain and water flowing into the lagoon from the drainage basin at 36 inflow points associated with small rivers and canals.
Floods have caused damage since ancient times and have become ever more frequent and intense as a result of the combined effect of eustatism (a rise in sea level) and subsidence (a drop in land level) caused by natural and man-induced phenomena. Today, towns and villages in the lagoon are an average of  lower with respect to the water level than at the beginning of the 1900s and each year, thousands of floods cause serious problems for the inhabitants as well as deterioration of architecture, urban structures and the ecosystem. Over the entire lagoon area, there is also a constant risk of an extreme catastrophic event such as that of 4November 1966 (the great flood) when a tide of  submerged Venice, Chioggia and the other built-up areas.
Floods effects are exacerbated due to greater erosion by the sea caused by human interventions to facilitate port activities (e.g. through the construction of jetties and artificial canals); establishment of the industrial Porto Marghera area; and increased wash from motorized boats, which all aggravate erosion of morphological structures and the foundations of quaysides and buildings.
In the future, the high water phenomenon may be further aggravated by the predicted rise in sea level as a result of global warming.
In this context, MOSE, together with reinforcement of the barrier island, has been designed to provide protection from tides of up to  in height. The aim of MOSE is to protect the lagoon, even if the most pessimistic hypotheses are proven true, such as a rise in sea level of at least .  However, the reports have grown more pessimistic with time compared to when MOSE was originally planned; the 2019 estimate from the IPCC (Intergovernmental Panel on Climate Change) predicts a rise in sea level of between  by 2100 if emissions continue to increase, which MOSE was not designed to handle.
MOSE is flexible and can be operated in different ways according to the characteristics and height of the tide. Given that the gates are independent and can be operated separately, all three inlets can be closed in the case of an exceptional event, the inlets can be closed one at a time according to the winds, atmospheric pressure and height of tide forecast, or again, each inlet can be partially closed.
Exceptionally high waters have struck the city since 1935: levels of 140 cm or greater have been recorded during the following floods, with 12 of the 20 events occurring in the 21st century:
Prior to 1936, the highest levels had been in 1879, when on 25 February the water reached 137.5 cm, and on 21 November 1916, when a level of 136 cm occurred. Since 1936, there have been 17 occasions when the level reached between 130 and 140 cm.
All values were recorded at the monitoring station at Punta della Salute (Punta della Dogana) and refer to the 1897 tidal datum point.
The highest tide in over five decades