Stole my opponent's skyshield t1 at the ESC.Generally when browsing through the internet, you see 2 kinds of lists: lists that don't consider deployment options like deepstrike, infiltrate, outflank etc., and lists that are built around the concept of alternative deployment. In my gaming philosophy, having the option to utilise things like deepstrike and infiltrate really open up ways to adapt into matchups, something that you might find super useful in a TAC list. However, over usage generally leads to gimmicky lists. I'd divide the effects of alternative deployment into three groups: (1) Giving board control outside your deployment zone before t1, (2) postponing the commitment of your assets and (3) opening up new attack vectors.
The first group is pretty simple, but can be game changing. Fleshhound spam utilises scout moves to already ensure midfield presence on t1 and reduces the distance required to cross towards enemy lines. Basically, you could say scouting takes away a whole shooting phase from your enemy. This is a pretty "crude" tactic in that it's pretty straightforward and one dimensional. Shadowsun + broadsides infiltrating can place your unit in better terrain, give you better shooting lanes or just place your unit withing range of your enemy, no matter how far back he deployed. I'd still say that the biggest advantage of gaining board control outside of your deployment zone from the get go, is in countering these sorts of moves, counter infiltration and scouting. You don't need anything flashy for this, in fact sacrificial units are often the best units for this purpose. Did anybody else notice the 13 flayed ones running around in Hesselberg's wraithwing in the Adepticon finals? he said he'd taken the unit partly for this exact purpose.
Postponing commitment takes a couple of forms. Rule number 1, if your unit has the infiltrate special rule, always infiltrate. If you're deploying first, or your opponent has infiltrators, you can wait and see how he deploys, and react to that. However, any unit can fit into this group, just by being placed in reserves. Your opponent has to react to the possibility of reserved units coming in from anywhere on your table edge (or elsewhere, depending on whether they can outflank/deepstrike). Jevgeni, the Russian Ork/Tau player, placed his riptide in regular reserve (spearhead deployment) during the last round of the ESC against me. This forced me to infiltrate the big kroot squad to block off a portion of the boardedge (6" in so he couldn't jump over), to ensure that he couldn't come on at an angle that wouldn't allow my broadsides cover against the str8 ap2 blast he had. Reserving always puts you at a disadvantage on the early turns, as you have less points on the table, so if you don't take this into account when building your lists, you might not be able to take advantage of reserves, even when it might give you a distinct advantage.
Have you ever deepstruck a riptide? In some matches I've opted to deepstrike him, bringing a pretty resilient threat somewhere in my opponent's backfield, without needing to first hop over his advancing elements. Deepstriking and outflanking can help you bypass parts of your opponent's army, and add new vectors with which to engage. Against some lists, this can shake up their playstyle/strategy. Andrew from Team USA raped our Tau player's backfield in a mirror match at the ESC with about 90 kroots coming on from Joonas' board edge (with the help of a positional relay on his riptide).
Most players would agree with the above ideas in theory, but find it to be less useful in practice. Then again, some people build lists totally around this concept (sometimes referred to as null deployment lists). In my opinion, alternative deployment is something to never count on (and shouldn't be used in every game) but completely neglecting them would be just as stupid. There ARE situations in which these tactics work really well, and trying to keep the option open when building your list adds versatility. It's all about balance.