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A well-selected amplifier located close to the antenna can improve the system noise margin by the (difference in noise figure of the amp vs the tuner) plus (all downstream insertion losses). That means that very weak signals otherwise just below the noise floor at the tuner's input can be raised enough to make them receivable.
As an example: Assume a tuner NF of 6 dB (probably fairly typical) and a preamp NF of 2 dB. Before the coax, one would already enjoy a 4 dB improvement in the system NF. Add in 50' of RG6 (3 dB) or 100' of RG6 (6 dB) or a 4-port splitter (8 dB) and it's illustrative as to how a weak signal which would otherwise lost in the system, can be preserved enough to be receivable.
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