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].
Second Wind's massive mid-fight health sustain allows you to absorb the brunt of your team's incoming damage when your [E] is up and come out the other side healthy enough to keep fighting.
The massive bonus health from Celestial Body contributes to the healing and shielding on Font of Life and Guardian.
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.