You get free boots at 10 min. but you cannot buy boots before then. These boots give additional MS. For each takedown you acquire the boots 30s sooner.
Braum naturally spends a lot of time near his teammates thanks to his [W] and [E]. The shield Provided by Guardian allows you and your AD Carry to absorb a bit of damage before you start blocking with [E].
WhyFont of Life
Between his Passive, [Q], and [R] Braum has a number of means of marking enemies for healing with Font of Life.
Conditioning on Braum makes you a mid-game front-line monster, able to block and absorb an incredible amount of damage for your team, and synergizes well with your [W].
Overgrowth's bonus health scaling is a great multiplier in defensive stats when paired with Conditioning. The extra bit of health will allow you to block and absorb more for your team.
Biscuit Delivery is a fantastic rune choice that will allow you to potentially take aggressive lane trade/duels while still being able to sustain afterward.
Magical Footwear is an interesting rune choice in that it can often be seen as a raw value of 350g allowing you to invest that into other items.