Political parties bring together people with broadly similar ideologies for the purposes of promoting a shared agenda in exerting political influence. The most important and conspicuous way in which they do this is by fielding candidates for public office. Although it is possible to stand for office independently, in practice the endorsement of a major party usually proves decisive. Even presidential candidates with substantial personal resources, such as Ross Perot, have failed to make much electoral impact without securing the Democrat or Republican nomination. More recently, Donald Trump, who combines personal wealth with a very high public profile, gained the Republican nomination for the presidency.
It was not clear before this which party Trump supported, as he had links with both, but it seems likely that the historical failures of candidates from outside the major political parties discouraged him from attempting to run on an independent platform.
Parties produce manifestoes in which their set out their policy positions and practical objectives. These change over time, often substantially, but at any given point the party manifesto articulates the current values of the party.
Parties also support their candidates in various ways, most substantially by raising funds, an activity which takes up a great deal of time and effort. In the case of the major parties, they also include an extensive bureaucracy, which provides administrative support for candidates.