IDs

Most AdWords entities expose a getId() method that returns their identifier. While not strictly necessary in most cases, IDs may come in handy when

Working with Reports
IDs provide for a good way to link a report row to the actual AdWords entity.
Maintaining a mapping with an external data store
You may already have ID-based information stored in your own database.
Looking for a bit of a performance boost
Fetching by IDs is often quicker than alternatives. The code for fetching a single entity is a bit easier too:
var campaigns = AdWordsApp.campaigns()
    .withIds([678678])
    .get();
// vs.
var campaigns = AdWordsApp.campaigns()
    .withCondition("Name='My Campaign'")
    .get();

Uniqueness

Campaign IDs and ad group IDs are unique: no two campaigns or ad groups will ever share the same ID. Ads and keywords, however, have composite IDs: a unique identifier of a keyword is a combination of its ad group ID and keyword ID. Likewise, a unique identifier of an ad is a combination of its ad group ID and ad ID. This has implications for the way selector.withIds() is called.

For campaigns and ad groups, selector.withIds() expects an array of numbers:

var ids = [123123, 234234, 345345];
var campaignSelector = AdWordsApp.campaigns().withIds(ids);

For ads and keywords, however, selector.withIds() needs an array of two-element arrays, the first element being the ad group ID. The following snippet retrieves three keywords from an ad group:

var adGroupId = 123123;
var keywordSelector = AdWordsApp.keywords().withIds([
    [adGroupId, 234234],
    [adGroupId, 345345],
    [adGroupId, 456456]
]);

Same construct applies when fetching ads.

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