Iron branches versus circlet to start the game
Why get iron branches (+1 to all attributes for 50 gold) when you plan to get a circlet (+2 to all attributes for 185 gold)?
Obviously cost might go into it a little bit, but you're saving gold and a courier trip in the long run if you get a circlet instead of 3 branches. You obviously also don't get that extra +1 to all attributes.
On heroes that build Bracers, Null Talismans or Wraith Bands, why wouldn't you get the circlet in lieu of your iron branches?
Solution 1:
One reason is Gold math. If you're buying some starting item(s) and regen, you might end up with roughly 100 Gold left. Which is enough for 2 Branches, but not for anything else. Now you might argue that you could just save that gold for later, but that's a bad idea in general because the +2 to all stats give you a much greater benefit than getting your later item 3 creeps earlier (also see below). Similar calculations apply if you're left with 50 or 150 Gold.
Another important reason is that Branches are extremely cost efficient and very space inefficient. They use up an entire slot for a mere +3 total boost, but cost just 17 Gold per stat, which is tremendously low. At the start of the game, you have little Gold and much space in your inventory, so Branches are a perfect fit.
Lastly, they turn out really, really strong when you compare advantages to disadvantages. Assuming you're not building them into anything and just selling them later, they actually cost you just 25 Gold. However, getting a +1 in each stat not only increases your survivability by a small amount (but at the start of the game, this small absolute value turns out to be not that small in relation to your stats out of the box), but most importantly increases your damage, making it easier to last hit and deny. 1 damage might not seem much, but if it costs you 25 Gold in the long run, it's really worth it. Chances are, you'll get at least 1 last hit you would have missed without them, and 1 is already enough to pay them off. That's not counting extra regen and extra harass damage.
If you even build them into Magic Wand or Mekansm, you get items that are very efficient and useful well into mid game, while preserving the stat gain from the original branches and restoring inventory space. Those are especially useful if you're not steamrolling the enemy team, so getting Branches early can help you get some comeback options slightly cheaper.
What it boils down to is that they're so damn cheap Gold wise that the only real reason not to get them is space constraints, which you usually don't have early.
Solution 2:
Limited inventory space is the primary reason you'd get a Circlet, which doesn't usually play much of a factor early on in the game. Iron Branches are significantly more effective per cost.
While Iron Branches only really build into a Magic Wand (and Mekanism), Magic Wand is an extremely good item and something most heroes build as the game goes on. Circlets on the other hand build into any of the three 3/3/6 stat items, which people rarely get more than one of, and almost never start with- it's not worth the recipe price.
Solution 3:
Regen is usually where you should be spending most of your starting gold, ensuring you can stay in the lane as long as possible and maximize farm.
For supports, starting gold is spent typically on things like courier and wards, picking up regen items second. Iron branches are a good way to fill up your remaining item slots.
Iron Branches also have a decent trade off, they don't cost much and you can sell them back when you need the inventory space.
They also are used for some good early game recipes like Headdress or Magic Wand, both of which offer some good benefits aside from stats.
The only thing Circlets are used to build are pure stat boosting items.