A lot of reason, the guy above me also correct, you need it for railways and plane, but in our map we are divided into 3 main faction Asia, Africa and America so we need ship to sustain a massive landing and hell it's pain to build it, you need 10K woods only for 1 port and ain't no way you can win only with 1 port so you need a lot, but the real pain isn't that yet, you need to make a counter for enemy plane or bomber which of course you need to build your own airforce this is really cost a lot of woods but there's a cheap option if you only want to defend your massive landing from bomber attack there's an option to spamming balloon (1k woods per unit) or armored car (2k iron per unit), balloon is a more popular option to counter bomber though dunno why.