Glass ionomer sealants are thought to prevent caries through a steady fluoride release over a prolonged period and the fissures are more resistant to demineralization, even after the visible loss of sealant material, however, a systemic review found no difference in caries development when GICs was used as a fissure sealing material compared to the conventional resin based sealants, in addition, it has less retention to the tooth structure than the resin based sealants.
These sealants have hydrophilic properties, allowing them to be an alternative of the hydrophobic resin in the generally wet oral cavity. Resin-based sealants are easily destroyed by saliva contamination.
They chemically bond with both enamel and dentin and do not necessarily require preparation/mechanical retention and can therefore be applied without harming existing tooth structure. This makes them ideal in many situations when tooth preservation is foremost and with minimally invasive techniques, particularly Class V fillings where there is a larger area of exposed dentin with only a thin ring of enamel. This often results in longer retention and service life than resin Class V fillings. 
They chemically bond to enamel and dentin leaving a smaller gap for bacteria to enter. Particularly when paired with silver diamine fluoride this can arrest caries and harden active caries and prevent further damage.
They can be placed and cured outside of clinical settings and do not require a curing light.
Chemically curable glass ionomer cements are considered safe from allergic reactions but a few have been reported with resin-based materials. Nevertheless, allergic reactions are very rarely associated with both sealants.