The role of judges in a common law legal system

A copy of the House of Lords' decision in Donoghue $v$ Stevenson $[1932]$ AC $562$ has been posted to wattle.

Even a 'skim' reading of this should give you some insight into the reasoning process of judges in a common law legal system. In Lord Atkin's judgment in particular, note the detailed reference to precedents, discussion and reasons which are presented as a justification for the decision $($final outcome$)$ in this case.

Specifically, in relation to Lord Atkin's judgment:

Identify any section in his Lordship's judgment which describes the central legal issue raised by the case;

Identify any section in his Lordship's judgment which describes the ratio of the case;

Numerous English cases are discussed, and even the United States legal position is referred to$,$ before Lord Atkin's final statement of legal liability in this case $-$ what is the purpose of this discussion and reference to other cases?