Yesterday I introduced my 5-year-old to the words <talk> and <walk>. He knows that graphemes can spell sounds, specifically that <l> can spell [l]. But he also knows that letters can perform other functions. Some letters are zeroed. Others are markers. The letter <l> can function as an etymological marker. Both <talk> and <walk> (along with <stalk>) are common English words in which <l> functions as an etymological marker.
The first word <talk> can spell [tɑk]. The second word <walk> can spell [wɑk]. The third word <stalk> can spell [stɑk]. In Englishes without the cot-caught merger, the <a> in all three words spells the phone [ɔ]. In none of the words does the <l> spell a phone. Instead, the <l> is an etymological marker in all three words.
Modern <talk> developed around 1200 from Middle English <talken>. Looking up the word on Etymonline, I find that <talk> is related to both <tale> and <tell>. All three words share a similar meaning related to expressing ideas and/or conversing. Etymonline identifies <talken> as a probable diminutive or frequentative form related to Middle English <tale>. Both <tale> and <tell> developed from the Proto-Indo-European <*del-> meaning “to recount, count.” The <l> in <talk> marks the etymology and the relationship to <tale> and <tell>.
Modern <walk> comes from Old English <wealcan> meaning “to toss, roll, move round” and <wealcian> meaning “to roll up, curl.” <walk> is related to the words <well> meaning “to rise up” and <welter> meaning “to roll or twist.” All three words convey a sense of movement. All three words also developed from PIE <*wel-> meaning “to turn, revolve.” The <l> in <walk> marks the etymology and the relationship to <well> and <welter>.
The word <stalk> is a homograph. One meaning is “the stem of a plant.” A second meaning is “to follow or approach stealthily.” The “plant stem” meaning likely arose as a diminutive of <stale>, which comes from Old English <stalu> meaning “wooden part.” The “pursue stealthily” meaning comes from Old English <-stealcian> as in <bestealcian> meaning “to steal along, walk warily” and is related to the word <steal>, which likewise conveys a sense of secrecy. The first <stalk> may also have influenced the second <stalk>. In both cases, the <l> in <stalk> marks the etymology and the relationship to <steal>.
If writing were sounds written down, <talk> could be spelled <*tak>, <walk> could be spelled <*wak>, and <stalk> could be spelled <*stak>. But writing is not just sounds written down. The primary function of English spelling is to represent meaning. Spelling also represents etymology, relationships, and phonology. The <l> in <talk>, <walk>, and <stalk> preserves the etymology. The <l> also preserves the relationships with other related words.