I think you're asking two questions, however in response to the backbencher one...
The role of a backbencher is to provide key information about their constituencies whilst they occupy an apprentice type role within government and are seen to be 'learning the ropes'.
If you look at the Wikipedia entry:
The term dates from 1855. A backbencher may be a new parliamentary
member yet to receive high office, a senior figure dropped from
government or someone who for whatever reason is not chosen to sit
either in the ministry or the opposition Shadow Ministry. By
extension, a backbencher is not a reliable supporter of all of their
party's goals and policies.
So historically it may be related to wanting to avoid unreliable members that could act against the designated direction of the party. They are seen as vital in informing decisions but not so much in taking them as they may hold an opposing view to that of the majority of party members. Historically it was probably a way of protecting the best(or vested) interests of the rest of the party members from representatives that were perhaps not yet indoctrinated in the importance of certain policies. Though this is only what I imagine. Roles such as the
chief whip could act to corroborate this view.
There is a good article one this website about the role of a backbencher and the powers that are allowed to them. Although this refers to the Australian system there is little difference that I see.
Backbenchers obviously have less impact on law-making than a minister
– yet there are a number of ways they can influence both the content
of legislation and the legislative process:
By serving on a parliamentary committee. This is perhaps where MPs on
the backbenches can have the most impact. Committees engage in
detailed scrutiny of existing legislation, hear from experts, consider
public submissions and ultimately make recommendations about suggested
changes to legislation. Some backbenchers with no leadership role in
the parliament or their party can take a lead role in these
committees, particularly on issues of great concern to them.
By participating in debate about bills. After the commencement of the
Second Reading, a date will be allocated for parliamentary debate
about a proposed bill. MPs can attend and speak for or against the
bill or its specific parts. Their speeches are recorded in Hansard,
which may be consulted by judges if the bill becomes legislation.
By suggesting amendments to a bill. During consideration in detail,
MPs who object to specific parts of a bill – but not to the bill as a
whole – may suggest amendments. Each amendment has to be voted on by
the house but, if accepted, it will become part of the bill.
By raising a private member’s bill. Not all bills begin with the
Cabinet or ministers. Any MP can introduce a bill into the parliament,
with or without the support of the government. Because of this,
private member’s bills are often voted down after the Second Reading
and rarely move beyond their house of origin. Only 15 private member’s
bills have been passed by the Federal parliament since 1901. The most
recent of these was the Euthanasia Laws Act (2007), introduced by
Liberal MP Kevin Andrews to overturn pro-euthanasia legislation passed
by the Northern Territory. The Federal government allowed a conscience
vote on the issue, rather than organising MPs to vote along party
By raising suggested law reforms in the party room. This is where all
MPs in a political party meet regularly to discuss matters pertaining
to parliament and policy. Individual MPs can float ideas and lobby for
changes to the law within their own party. These proposals may or may
not be accepted and acted upon by ministers and the Cabinet.
From this we can see that they can affect change within parliament however less than frontbenchers.
Hope this helps!