A special blessing is associated with dew (tal) and not with rainfall (matar). When Yitzchak blessed his son Yaakov, he said, “May G-d give you of the dew of the heavens” (Bereishit 27:28). The blessing did not mention the rain of the heavens, because rain sometimes makes people unhappy. In contrast, everybody is happy with dew. Furthermore, dew appears all year long, without fail. Perhaps because it is ever-present, many do not mention it in the second blessing of the Amidah (as a parallel to the mention of rain, “mashiv haru’ach”). Others do mention dew during the summer months, inserting “morid ha-tal” in the second blessing.
Nevertheless, everyone includes dew in a later blessing in the Amidah, when we petition G-d: “ve-ten tal u-matar li-verachah.” The need to specify that we want dew and rain for blessing (li-verachah) may be because dew is not always a blessing. For example, if a person places fruit on his roof to dry them out and get rid of any insects inside them, he does not want dew to wet the fruit. Though dew is one of the seven liquids that normally make fruit susceptible to defilement (mekabel tum’ah), nevertheless since the person who placed the fruit on the roof does not want dew to form on his fruit, they do not become susceptible.
The laws pertaining to dew and water are the same in almost all ways (including causing susceptibility to defilement). Nevertheless, dew is considered a liquid in its own right. Thus in the list of seven liquids that make crops susceptible to defilement (wine, honey, oil, milk, dew, blood, and water with the Hebrew mnemonic “yad shachat dam”), water and dew are listed separately. This is because there are some differences between the two. For example, the amount of water a person would need to transport in order to be liable for carrying on Shabbat is different from the amount of dew that would make him liable.