The procedure in the British parliament differs in regard to the preambles of public and private bills. The second reading of a public bill affirms the principle, and therefore in committee the preamble stands postponed until after the consideration of the clauses, when it is considered in reference to those clauses as amended and altered if need be (Standing Order 35). On the other hand, the preamble of a private bill, if opposed, is considered first in committee, and counsel for the bill deals with the expediency of the bill, calls witnesses for the allegation in the preamble, and petitions against the bill are then heard; if the preamble is negatived the bill is dropped, if affirmed it is gone through clause by clause. On unopposed private bills the preamble has also to be proved, more especially with regard to whether the clauses required by the standing orders are inserted (see May, Parliamentary Practice, 1906, pp. 483, 808 seq.).
from a 1911 encyclopedia
While preambles of legal texts may seem just like unimportant introductory matter, their wording may have effects that may not have been foreseen by their drafters. For instance, it's on the basis of the preamble of the French Constitution, mentioning the solemn regard of the French Republic towards the principles set forth in the 1789 Declaration of the Rights of Man and of the Citizen that the Constitutional Council has declared certain laws to be unconstitutional (the first case being decision 71-44DC). For this reason, the redaction of the preamble of the proposed European Constitution, in 2002, has caused much controversy because of the possible inclusion of references to the "Christian heritage" of Europe; could such a sentence be used in the future from a legal point of view?