Rabbit Hole: for when you absolutely, positively, need 3 right this second.
If you put in 3x Rabbit Hole (which is easy, even out-of-faction, due to the 1 influence cost) then you will find the first one quite early; you have about a 50% chance of seeing at least one in your first 9 cards (in a 45 card deck). Then, for 6 and one click to install, you should typically be able to pull them all from your deck at once, unless you got unlucky and have two in your opening hand.
This is the most efficient way to get three link in the game. Playing three Access to Globalsec costs you three cards, three clicks to install, and three credits, and that's assuming you can even find all three before the end of the game. Dyson Mem Chip is the same way, although you're getting some along the way. But that's still very, very slow, and if your gameplan relies on being linked to the max, it simply won't cut it.
But...why? Who on earth needs that much link strength?
For a long time, the answer was "pretty much nobody". A number of runners (Kate, Ed, Andy, and "Geist") start off with one link, and a single Dyson Mem Chip would put you in range to get free money from Underworld Contact and free MU from cloud breakers. On top of that, most decks didn't really run traces. You might see a few here and there, particularly in kill decks due to Midseason Replacements, SEA Source, or Punitive Counterstrike, but tracer ICE wasn't too popular. Notable exceptions to this were Data Raven and Ichi 1.0, but in both cases the trace was pretty secondary, and pretty weak anyway.
But Data and Destiny has two things which will change this up. The first is a huge boost to tracer ICE, in the form of the agenda Improved Tracers and the current Surveillance Sweep. With these two cards active, a lot of ignored ICE has the potential to become really dangerous; three of the four snake ICE (Viper, Caduceus, Uroboros) become really gross for someone without link, as do all the Zodiac ICE (Gemini, Taurus, Sagittarius, and Virgo). This meta shift might be enough to make link worthwhile on it's own.
The second D&D change is the introduction of Sunny Lebeau: Security Specialist, with her cards Security Nexus and Security Chip. These cards make link more valuable against any matchup--if you can efficiently beat a trace 5, you get a free Inside Job every turn, and the Chip gives your breakers the strength they need to chew through just about anything. Sunny starts off with two link, and the Nexus makes three; but to really capitalize on the Nexus and the Chip, you want as much link as you can get. Rabbit Hole fits that niche nicely.