Damage Dealer
This is probably the most common and most used of them all. Samurais are best at gaining TP fast and being able to put out fairly good WS numbers at a quick pace. It is this fact which is why many seek to design 5-hit and 6-hit builds over many others. For damage-to-TP given ratio, I would say SAM are high up (bested by gun/bow RNGs, BLMs). Other than those points, I don't think SAM are anything special as a damage dealer.
Tank
This is the second most underrated role (in my opinion). There are a handful of people who do use this to the full extent. A Samurai has access to Seigan, which can be fairly reliable if used correctly. That is also supported by access to a nice set of equipment that lowers damage. A competent and well-geared Samurai can be able to tank all sorts of things like lesser HNMs, Sky Gods, alternate versions of HNMs (like Nyzul and ZNM versions), and even chariots. In another post, I will discuss more about tanking these sorts of things. As a tank, a Samurai does not really need NIN sub or a WAR sub, as they can tank purely through damage and Seigan. The one benefit of a SAM over PLD/NIN or NIN tank is that any magical AoE (either spells or TP moves based on magic) will not wipe shadows, whereas utsusemi will be wiped. On some fights, this can be a very beneficial thing.
Hate Control
With a fairly potent and accurate WS, comes the hate control with a THF sub. Samurais can use trick attack to put a nice amount of damage onto a different target and be able to keep up with the 1-minute timer. If done from behind, a Samurai has Sneak Attack which will provide a large boost to the damage. If done from the front, a Samurai has Overwhelm which can provide up to an additional 19% damage bonus from merits.
Ranged Attacker
This is one that has been seen in less and less use over time. A SAM/RNG has access to Sidewinder (the most powerful Archery WS) coupled with a C+ skill in Archery. Samurais also have some decent bows (only 3-4 bows out of the top ones are RNG only), decent selection in gear for Ranged Accuracy, and a subpar selection for Ranged Attack. They even have the ability to wield Yoichinoyumi. The benefit from being a ranged attacker is being outside the range of most AoE attacks, as well as being able to continue to attack in a kited-fight. With current strategies (such as TP-burn Kirin), this is very rarely seen nowadays.
Healer
This is the most underrated role (in my opinion), though a very niche one as well. A SAM/DNC has Curing Waltz I&II, Divine Waltz, Healing Waltz, and Drain Samba I&II. These dances consume TP which is a Samurai's forte. The biggest pitfall of a Samurai healer is it cannot keep up with spike damage (like TP moves that land for 800+ ) or for high Damage over Time. A SAM/DNC can provide as a support healer or even main heal in some low-man situations.
Soloer
Not really a role, but a Samurai can basically couple as a Healer, a Tank, and a Damage Dealer to become somewhat of a soloer. They can solo a wide variety of NMs, or even some BCNMs like Operation Desert Storm. Or if they really wanted to, they can solo chain Ts and some VTs.
Versatility
The greatest asset to a Samurai is that with proper preparation, you can easily change roles. One can easily jump from being a damage dealer to a tank by simply swapping gears and putting up Seigan (as well as gain hate if necessary). Natively, a Samurai can be a damage dealer, tank, or ranged attacker as long as they have the suitable equipment for it. Very few jobs have such a luxury as a Samurai in that regard.
Progress Report

Finally finished with the last two merits. With parrying skill done, it frees up some time, some of which I can use to finish my work with parrying and -damage taken set. I have the materials for Gavial Greaves+1 but just never had my crafter do them yet. As far as parrying goes, I still have to get Tomoe, and upgrade the AF1 legs (been sitting on Apollyon piece for some time).
Today's Random Fact
You cannot use elemental WSes to skill up on a combat skill.


