Published manifestos-a document that outlines the parties policies, even when theres no election, they still look for new policies.
They're an organisational structure needed to form a gov; winning elections and forming a gov requires lots of planning. Gov will work hard to keep mps united and organised, eg the whipping system.
Parties provide a link between parliament and the people- they all have constituencies with an mp from their local area. Local activists regularly meet their mp to highlight concerns in the area and this mps needs to maintain the link between voters and the elected office.
A party's ideologyhelps with voters who dont understand politics make choices as they'll make assumptions about what policies they may enact if elected.
Nurseries for future politicians-many mps have previously worked or volunteered for their local or national party organisation, giving them majority of their political knowledge.
Parties allow people to become active in their areas politics and help their local mp.
Aggregating society desires-people want different things from the government. It's the parties task to bring together these competing interests and create a policy programme that pleases as many as possible. The result of this will show up in elections. The electoral math demands it.
EG TB "tough on crime"- stricter punishments for offenders was the answer to rising crime and others felt it was poverty. Labour appealed to both views.