In Brooklyn, letters don't just transmit words—they transmit memories.
After all, when Eilis receives a letter, very little focus is placed on its actual contents. Instead, Eilis is given the opportunity to think about the people she loves, like when she reads her brother's letter and "could hear Jack's voice in the words he wrote" and "feel him in the room with her" (3.787). In other words, Eilis is less interested in hearing about the day-to-day minutiae of her family's life than simply feeling their presence.
Of course, this little memory trip can be overwhelming at times, leading Eilis to avoid reading letters when she knows that the emotional punch will be too powerful. Ultimately, however, this just further emphasizes how much Eilis is affected by the letters she receives.