Why is scrutiny of the executive effective?

Parliamentary questions:an opportunity for MPs and Lords to question government ministers about matters for which they are responsible.
This usually entails more detailed questioning and ministers are given notice of oral questions so that they can prepare with the assistance of civil servants. MPs can also submit written questions that are answered by civil servants.
This allows opposition MPs to inform themselves about government policy, and individual members can raise issues of interest to their constituents.
This is, of course, much less well-known than the highly theatrical verbal duels between the prime minister and the leader of the opposition.
PMQs: Leader of the opposition, leader of the third largest party and backbenchers selected by the speaker can question PM on any matter – in 2015 Corbyn asked questions sent in by the public.

Have proven more effective than debates at scrutinising executive and holding it to account.
Scrutinise policy, administration and expenditure of the government.
They reflect the party balance in the HofC and spend time questioning ministers, officials and outside experts and can request access to government papers.